/ 
Database-Join-0.005.0
/ README.md

NAME

Database::Join - Read-only combined view across two or more Database::Abstraction objects

VERSION

Version 0.004.0

SYNOPSIS

Basic two-database join

use Database::Join;

# Step 1: create each component database the normal way
my $customers = Database::Customers->new(directory => '/data');
my $loyalty   = Database::Loyalty->new(directory  => '/data');

# Step 2: combine them on the shared key column 'entry'
my $join = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
);

# Step 3: query exactly as you would a single Database::Abstraction object
my $all_rows  = $join->selectall_arrayref();
my $vip_rows  = $join->selectall_arrayref(tier => 'gold');
my $one_row   = $join->fetchrow_hashref(entry => 'C001');
my $total     = $join->count();
my $col_names = $join->columns();

Hiding internal columns

my $join = Database::Join->new(
    databases      => [ $customers, $loyalty ],
    join_column    => 'entry',
    remove_columns => [ 'internal_id', 'audit_ts' ],
);
# 'internal_id' and 'audit_ts' never appear in results or columns()

join_map: when the key column has different names in each database

# $cities  (index 0) has a column called 'statecode' -- matches join_column
# $stnames (index 1) has a column called 'entry'     -- different name

my $join = Database::Join->new(
    databases   => [ $cities,  $stnames ],
    #                index 0   index 1
    join_column => 'statecode',
    join_map    => { 1 => 'entry' },  # index 1 calls its join key 'entry'
);

# All returned rows use 'statecode'; 'entry' is never exposed
my $rows = $join->selectall_arrayref();

filters: permanently restrict a database's visible rows

# Only show orders placed more than 60 days ago, without repeating
# the criterion on every query call.
my $join = Database::Join->new(
    databases   => [ $customers, $orders ],
    join_column => 'entry',
    filters     => { 1 => { age_days => { '>' => 60 } } },
);

my $rows = $join->selectall_arrayref();                 # all old orders
my $vip  = $join->selectall_arrayref(tier => 'gold');   # old + gold tier

Inner and outer join types

my $inner = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
    join_type   => 'inner',   # only keys present in BOTH databases
);

my $outer = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
    join_type   => 'outer',   # all keys from EITHER database
);

Building the view incrementally with add_database

my $join = Database::Join->new(
    databases   => [ $customers ],
    join_column => 'entry',
);

$join->add_database($loyalty)
     ->add_database($scores, remove_columns => ['raw_score']);

AUTOLOAD column shortcut

# Returns the 'name' value for entry 'C001' (scalar context)
my $name = $join->name(entry => 'C001');

# Returns all 'tier' values (list context)
my @tiers = $join->tier();

SQLite join backend for large datasets

# 'auto' (default): switches to SQLite automatically above the threshold
my $join = Database::Join->new(
    databases      => [ $customers, $loyalty ],
    join_column    => 'entry',
    backend        => 'auto',          # default
    max_array_rows => 50_000,          # use SQLite when combined rows > 50,000
);

# Always use SQLite -- useful when you know the data is large
my $join = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
    backend     => 'sqlite',
    tmpdir      => '/fast/nvme/tmp',   # optional: faster temp disk
);

# Always use the original in-memory path
my $join = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
    backend     => 'array',
);

DESCRIPTION

Database::Join merges two or more Database::Abstraction objects into a single logical, read-only view. Each component database is queried independently through its own Database::Abstraction interface. The results are combined using a shared key column (join_column). In effect, this means that you can view data from more than one database using an intuitive, non-SQL interface.

The module exposes the same read-only API as Database::Abstraction: selectall_arrayref, selectall_array, fetchrow_hashref, count, columns, schema, updated, set_logger, and the AUTOLOAD column shortcut. Callers do not need to know how many underlying databases are involved.

Think of it as a virtual database table that is assembled on demand from several real tables, one per component database.

Join backends

By default (backend => 'auto'), Database::Join first checks the combined source row count. For small datasets (up to max_array_rows, default 10,000 rows) it merges entirely in Perl memory. For larger datasets it automatically spills source rows into a temporary SQLite database and executes a single SQL JOIN there, keeping peak RAM to roughly one times the source data size instead of three. You can also force either path unconditionally with backend => 'sqlite' or backend => 'array'.

Join semantics

The join_type parameter controls what happens when a particular key value exists in some component databases but not all:

Important override rule: whenever you pass a query criterion for a column that belongs to a secondary database, that database automatically acts as an inner-join partner for that query only -- regardless of join_type. This gives WHERE-clause semantics. For example, if you have a LEFT join but query tier => 'gold' on a secondary database, only rows whose secondary entry has tier = 'gold' are returned (rows with no secondary entry are excluded, just as a WHERE clause would exclude them).

Column ownership and routing

At construction time, Database::Join calls columns() on each component database and builds an internal index that maps every column name to the database that owns it.

When you pass criteria to a query method, each key-value pair is automatically routed to the right database. You never need to say which database a column belongs to.

The join_column is special: criteria on it are broadcast to all databases so that each database fetches only the relevant rows before the in-memory merge.

When the same non-join column name exists in more than one database, the last database in the databases array wins by default: its value overwrites earlier ones in merged rows. Use collision_prefix to preserve both values under distinct names instead.

LIMITATIONS

COMMON PITFALLS

METHODS

new

SYNOPSIS

my $join = Database::Join->new(
    databases        => [ $db1, $db2 ],
    join_column      => 'entry',
    join_type        => 'left',
    join_map         => { 1 => 'local_col' },
    filters          => { 1 => { score => { '>' => 60 } } },
    collision_prefix => { 1 => 'right' },
    remove_columns   => [ 'email', 'internal_id' ],
    backend          => 'auto',        # 'auto' | 'sqlite' | 'array'
    max_array_rows   => 10_000,        # threshold for 'auto' mode
    tmpdir           => '/tmp',        # directory for temp SQLite file
    logger           => $log,
    i18n             => $locale,
);

DESCRIPTION

Constructs and returns a new Database::Join object.

Each element of databases must be an already-instantiated subclass of Database::Abstraction. The constructor calls columns() on every database to build an internal column-routing table and verifies that join_column (or its local alias from join_map) is present in each one.

Columns listed in remove_columns are hidden immediately: they do not appear in columns(), schema(), or any returned row hashref. This is equivalent to calling remove_column once per name after construction.

API SPECIFICATION

Input

databases      => { type => 'arrayref', required => 1 }
                  # One or more Database::Abstraction subclass objects.
                  #
                  # DOMAIN -- EP valid:   non-empty arrayref of blessed DA subclasses.
                  # DOMAIN -- EP invalid: scalar, hashref, or absent => croak.
                  # DOMAIN -- BVA size:   minimum 1 element; no documented upper bound.
                  # DOMAIN -- BVA elem:   each element must pass isa('Database::Abstraction').

join_column    => { type => 'string',   optional => 1, default => 'entry' }
                  # The column name shared by all databases (the join key).
                  #
                  # DOMAIN -- EP valid:   any non-empty string present in every component DA.
                  # DOMAIN -- EP invalid: column absent from any DA => croak join_col_missing.
                  # DOMAIN -- BVA:        empty string '' is treated as a column name and
                  #                       will croak if (as expected) it is absent from every DA.
                  # DOMAIN -- NOTE:       matching is case-sensitive and exact.

join_type      => { type => 'string',   optional => 1, default => 'left',
                    enum => ['inner', 'left', 'outer'] }
                  # Controls which keys appear in the result when not all
                  # databases share the same key values.
                  #
                  # DOMAIN -- EP valid:   exactly 'inner', 'left', or 'outer'.
                  # DOMAIN -- EP invalid: any other string including 'INNER', 'LEFT',
                  #                       'OUTER' (enum check is case-sensitive), 'cross',
                  #                       or '' => croak from validate_strict.

join_map       => { type => 'hashref',  optional => 1 }
                  # Zero-based database index => local column name.
                  # See the join_map section for full details.
                  #
                  # DOMAIN -- EP valid:   hashref values must be plain strings.
                  # DOMAIN -- EP invalid: reference value (hashref, arrayref, coderef, etc.)
                  #                       => croak; the guard prevents heap-address leakage.
                  # DOMAIN -- BVA:        out-of-range keys (beyond the databases array) are
                  #                       silently ignored.

filters        => { type => 'hashref',  optional => 1 }
                  # Zero-based database index => criteria hashref.
                  # Permanent row restrictions on individual databases.
                  # See the filters section for full details.

collision_prefix => { type => 'hashref', optional => 1 }
                  # Zero-based database index (>0) => prefix string.
                  # When a secondary database has a column that collides
                  # with a column already present in the merged view, the
                  # secondary column is published as "$prefix.$col" instead
                  # of silently overwriting the earlier value.
                  # Index 0 entries are silently ignored.
                  # Omitting this parameter preserves the original
                  # last-database-wins behaviour.
                  # See the collision_prefix section for full details.
                  #
                  # DOMAIN -- EP valid:   absent or {} => last-database-wins (no change).
                  # DOMAIN -- EP valid:   { N => 'prefix' } where N > 0 => colliding
                  #                       columns from DB[N] published as "$prefix.$col";
                  #                       non-colliding columns from the same DB added plain.
                  # DOMAIN -- EP note:    index-0 entries are silently ignored.
                  # DOMAIN -- Invariant:  join_column is never prefixed regardless of
                  #                       collision_prefix configuration.

remove_columns => { type => 'arrayref', optional => 1 }
                  # Column names to hide from the merged view.
                  #
                  # DOMAIN -- EP valid:   arrayref of any strings; non-existent columns
                  #                       are silently ignored (idempotent).
                  # DOMAIN -- EP invalid: join_column itself => croak remove_join_col.
                  # DOMAIN -- BVA:        [] empty arrayref is a safe no-op.

backend        => { type => 'string',   optional => 1, default => 'auto',
                    enum => ['array', 'sqlite', 'auto'] }
                  # Controls which join strategy is used.
                  #   'auto'   -- (default) use 'array' when combined source row count
                  #              <= max_array_rows, 'sqlite' otherwise.
                  #   'sqlite' -- always spill to a temporary SQLite database.
                  #   'array'  -- always use the in-memory merge path.
                  #
                  # DOMAIN -- EP valid:   'array', 'sqlite', or 'auto' (case-sensitive).
                  # DOMAIN -- EP invalid: any other string => croak error_invalid_backend.
                  # DOMAIN -- Default:    'auto'.

max_array_rows => { type => 'integer',  optional => 1, default => 10_000 }
                  # Row-count threshold for 'auto' mode.  When the combined
                  # source row count exceeds this value, the SQLite path is used.
                  # Ignored when backend is 'array' or 'sqlite'.
                  #
                  # DOMAIN -- EP valid:   any non-negative integer.
                  # DOMAIN -- BVA:        0 means always use SQLite (all counts exceed 0).
                  # DOMAIN -- Default:    10,000.

tmpdir         => { type => 'string',   optional => 1 }
                  # Directory for the per-call temporary SQLite database file.
                  # The file is created securely by File::Temp and removed when
                  # the query completes.  Ignored when backend is 'array'.
                  #
                  # DOMAIN -- EP valid:   any writable directory path string.
                  # DOMAIN -- EP absent:  uses File::Spec->tmpdir() (system temp dir).

logger         => { type => 'object',   optional => 1 }
                  # Logger object propagated to all component databases.

i18n           => { type => 'object',   optional => 1 }
                  # Localisation object with a translate($key, @args) method.

Output

A blessed Database::Join object.

EXAMPLE

# Customers database: entry | name | email
# Loyalty   database: entry | tier | points

my $join = Database::Join->new(
    databases      => [ $customers, $loyalty ],
    join_column    => 'entry',
    join_type      => 'inner',            # only customers who also have loyalty records
    remove_columns => [ 'email' ],        # hide PII from query results
    filters        => { 1 => { points => { '>' => 0 } } }, # ignore zero-point records
);

my $rows = $join->selectall_arrayref();
# Each row: { entry => ..., name => ..., tier => ..., points => ... }
# 'email' is absent. Zero-point loyalty records are excluded.

PSEUDOCODE

validate all parameters with validate_strict
croak if databases is empty
croak if any element of databases is not a Database::Abstraction subclass
bless the object with all fields initialised
call _build_col_index to map every column to its owning database
    and verify join_column presence in each database
for each column in remove_columns: call remove_column
return the new object

MESSAGES

error_no_databases     -- databases arrayref was empty
error_invalid_db       -- an element of databases is not a D::A subclass
error_join_col_missing -- join_column (or its join_map alias) not found in a database
error_invalid_backend  -- backend value is not 'array', 'sqlite', or 'auto'
error_sqlite_connect   -- temporary SQLite database could not be created (backend='sqlite'/'auto')

join_map - joining on differently-named columns

By default every component database must have a column whose name matches join_column. If a database uses a different local name for the join key, declare the mapping with join_map.

join_map is a hashref. Each key is the zero-based position of a database in the databases array (0 = first, 1 = second, and so on). Each value is the name that that particular database uses for the join key.

Databases not listed in join_map are assumed to already have a column named join_column and need no entry.

Throughout the merged view the join key is always referred to by the name given in join_column. The local alias is never exposed in returned rows, in columns(), or in schema().

When do you need join_map?

You need join_map when you have two tables like:

cities table  : entry (the city name) | statecode
stnames table : entry (the state code) | state

Here you want to join cities.statecode to stnames.entry. You choose join_column => 'statecode' as the canonical name, but stnames calls that same concept entry, so you declare:

join_map => { 1 => 'entry' }  # stnames (index 1) calls it 'entry'

Example

#                        index 0     index 1
my @databases = (       $cities,    $stnames  );
#  join key column:    'statecode'  'entry'
#  join_column:        'statecode' (chosen canonical name)
#  stnames differs, so declare the alias:

my $join = Database::Join->new(
    databases   => \@databases,
    join_column => 'statecode',
    join_map    => { 1 => 'entry' },
);

my $rows = $join->selectall_arrayref();
# Each $row has keys: entry (city), statecode, state
# 'entry' from stnames is never exposed directly.

my $row = $join->fetchrow_hashref(statecode => 'CA');

Using add_database instead

If you build the join incrementally with add_database, pass join_column directly to that call instead of using join_map:

my $join = Database::Join->new(
    databases   => [ $cities ],
    join_column => 'statecode',
);
$join->add_database($stnames, join_column => 'entry');

This is exactly equivalent to the join_map form above.

filters - permanent per-database row filters

filters lets you restrict a component database to a subset of its rows permanently, without repeating the criterion on every query call.

Think of it as telling the join: "whenever you query this database, always add these extra conditions". Callers never need to specify the restriction themselves and can never accidentally omit it.

filters is a hashref. Each key is the zero-based position of a database in the databases array (same numbering as join_map). Each value is a criteria hashref in the same format as selectall_arrayref accepts.

Key-set semantics

A filtered database always acts as an inner-join partner, regardless of the join_type setting. Any join-key value that does not pass the filter is excluded from the merged output entirely -- not just missing its secondary columns. This ensures the filter genuinely restricts the view rather than simply hiding a few fields.

Criteria merging

When a query call also passes a criterion for a column that already has a base filter, the two constraints are combined:

Example -- only show orders placed more than 60 days ago

my $join = Database::Join->new(
    databases   => [ $customers, $orders ],
    join_column => 'entry',
    filters     => { 1 => { age_days => { '>' => 60 } } },
);

# Every query automatically sees only old orders
my $rows = $join->selectall_arrayref();

# Additional criteria layer on top -- gold tier AND old order
my $vip  = $join->selectall_arrayref(tier => 'gold');

# Range intersection: age_days > 60 AND age_days < 365
my $mid  = $join->selectall_arrayref(age_days => { '<' => 365 });

When using add_database, pass filter (singular) to set the base criteria for the new database:

$join->add_database($orders, filter => { age_days => { '>' => 60 } });

collision_prefix - preserve colliding columns from secondary databases

By default, when a column name appears in more than one database the last database wins: its value silently overwrites earlier ones in merged rows. This loses data and makes the origin invisible.

collision_prefix changes this for secondary databases you designate. When a secondary database at index N has a column that already exists in the merged view, and collision_prefix->{N} is set, the colliding column is published as "$prefix.$col" instead of overwriting. Both values are then visible: the original column keeps its name (from the earlier database), and the collision gets the prefixed name.

collision_prefix is a hashref. Each key is the zero-based index of a secondary database in the databases array (same numbering as join_map). Each value is the prefix string to prepend. An index-0 entry is meaningless and silently ignored. Omitting collision_prefix entirely preserves the previous last-wins behaviour and changes nothing.

Non-colliding columns from a secondary database are always added as-is with no prefix, whether or not collision_prefix is configured.

Example -- sales table and products table, both with a "product" column

# $sales    columns: id, product, amount, date
# $products columns: sku, product, price, category
# join on 'product' (left key) matched against 'sku' (right key via join_map)

my $join = Database::Join->new(
    databases        => [$sales, $products],
    join_column      => 'product',
    join_map         => { 1 => 'sku' },
    collision_prefix => { 1 => 'products' },
);

$join->columns;
# => ['amount', 'category', 'date', 'id', 'price', 'product', 'products.product']
#                                                               ^-- prefixed collision

my $row = $join->fetchrow_hashref(product => 'widget');
# $row->{product}            -- value from $sales
# $row->{'products.product'} -- value from $products (different row, same column name)
# $row->{price}              -- from $products, no collision, kept as-is

Querying on a prefixed column

Use the full published name as the criterion key:

my $rows = $join->selectall_arrayref('products.product' => 'widget');
# Internally routes as: product => 'widget' to $products

Interaction with remove_column

remove_column operates on published names. To suppress a prefixed collision column entirely, pass the prefixed name:

$join->remove_column('products.product');

backend - SQLite join backend for large datasets

Database::Join can merge component databases in two different ways, controlled by the backend constructor parameter.

Choosing max_array_rows

The default of 10,000 is a reasonable starting point. Adjust it to match your hardware and typical row width. For wide rows (many columns or long strings) you may want a lower threshold; for narrow rows you can raise it.

Temporary file: name, location, and lifetime

When the SQLite path is active, a single temporary SQLite database file acts as the join cache for the life of the Database::Join object.

Name: the filename is randomly generated by File::Temp, for example:

/tmp/Cj8xK7mP2Q.db

The random portion (ten characters) is chosen automatically to avoid collisions. Only the directory is under your control; you cannot specify the filename itself.

Location: controlled by the tmpdir constructor parameter. If tmpdir is not specified, File::Spec->tmpdir() is used (usually /tmp on Unix, or the value of the TEMP or TMP environment variable on Windows).

Lifetime: one file per object, deleted when the object is destroyed: the file is created on the first query call that uses the SQLite path and kept alive until the Database::Join object is destroyed (i.e. when it goes out of scope or is explicitly undef-d). At most one file exists per object at any given moment. Calling selectall_arrayref() ten times on the same object creates and uses one file, not ten.

Cache invalidation: the cache is automatically rebuilt (the old file is replaced with a new one) when any source database's updated() return value changes, or when add_database() is called. Base-filter criteria (filters constructor parameter) are applied once at build time for spilled sources; query-time criteria are applied per-call as SQL WHERE clauses.

To use a different directory -- for example a RAM-backed filesystem or a faster local disk:

my $join = Database::Join->new(
    databases      => [ $db1, $db2 ],
    join_column    => 'entry',
    backend        => 'sqlite',
    tmpdir         => '/dev/shm',     # Linux RAM disk
);

Zero-copy ATTACH (dbi_source() interface)

Normally, when the SQLite path is active, rows from each component database are fetched one by one and inserted into the temporary SQLite file. This is efficient but does involve INSERT overhead.

If a component database is itself SQLite-backed and implements a dbi_source() method, Database::Join can skip the row-by-row copy entirely and instead use ATTACH DATABASE to link the source file directly to the temporary join connection. This is the zero-copy path and is significantly faster for large SQLite sources.

The dbi_source() method must return a hashref with two keys:

Example implementation:

package My::SQLiteDatabase;
use parent 'Database::Abstraction';

sub dbi_source {
    my ($self) = @_;
    return {
        dbh   => $self->{_dbh},      # connected DBD::SQLite handle
        table => $self->{_table},    # table name in that database
    };
}

1;

The zero-copy ATTACH path is only used when there are no query-time criteria for that database in the current call. When criteria exist, the database is queried via selectall_arrayref as usual and the resulting rows are inserted into the temporary file.

Result identity

Both the array path and the SQLite path produce identical results for any given query. You can switch between them freely without changing callers. The collision_prefix column renaming, join_map key translation, and all three join types (left, inner, outer) work identically on both paths.

selectall_arrayref

SYNOPSIS

my $rows = $join->selectall_arrayref();
my $rows = $join->selectall_arrayref(tier  => 'gold');
my $rows = $join->selectall_arrayref(score => { '>' => 80 });
my $rows = $join->selectall_arrayref('C001');  # positional: entry => 'C001'

DESCRIPTION

Returns an arrayref of hashrefs representing the merged view of all component databases, optionally filtered by the given criteria.

Criteria for columns that live in different databases are routed automatically: each database is queried with only the criteria that apply to its own columns. The results are combined in memory using join_column.

Accepts the same criteria syntax as Database::Abstraction::selectall_arrayref. A single plain scalar argument is interpreted as the join_column value (equivalent to entry => 'C001' when join_column is 'entry').

API SPECIFICATION

Input

Calling conventions (in order of precedence):
  1. No arguments             -- returns all rows
  2. One plain scalar         -- shorthand for join_column => $scalar
  3. Key-value pairs or
     a criteria hashref       -- routed per-database

Values may be:
  Plain scalar                -- exact match
  Hashref of operators        -- e.g. { '>' => 80 }

Output

Arrayref of hashrefs; one hashref per qualifying merged row,
sorted ascending by join_column value.
Returns a reference to an empty array when no rows match.

EXAMPLE

# All rows from both databases
my $all = $join->selectall_arrayref();

# Only rows where the 'tier' column (from the loyalty database)
# equals 'gold' -- the criterion is routed to the right database
my $vip = $join->selectall_arrayref(tier => 'gold');

# Operator hashref: score > 80
my $high = $join->selectall_arrayref(score => { '>' => 80 });

# Access each merged row
for my $row (@{$vip}) {
    printf "%-10s tier=%-8s score=%d\n",
        $row->{entry}, $row->{tier}, $row->{score} // 0;
}

selectall_array

SYNOPSIS

my @rows = $join->selectall_array(tier => 'gold');

# Scalar context: only the first matching row
my $first = $join->selectall_array(entry => 'C001');

DESCRIPTION

In list context returns a list of merged hashrefs -- the same rows that selectall_arrayref would return, just as a flat list rather than an arrayref.

In scalar context returns only the first matching hashref (or undef if nothing matches).

API SPECIFICATION

Input

Same as selectall_arrayref.

Output

List context:   list of hashrefs (may be empty).
Scalar context: single hashref or undef.

EXAMPLE

my @all = $join->selectall_array();
print scalar @all, " rows\n";

# First gold-tier customer only
my $first_vip = $join->selectall_array(tier => 'gold');
print $first_vip->{name}, "\n" if defined $first_vip;

fetchrow_hashref

SYNOPSIS

my $row = $join->fetchrow_hashref(entry => 'C001');
my $row = $join->fetchrow_hashref('C001');   # positional shorthand

DESCRIPTION

Returns a single merged hashref for the first row matching the given criteria, or undef when nothing matches.

Equivalent to calling selectall_arrayref and taking only the first element. All the same criteria conventions apply.

API SPECIFICATION

Input

Same as selectall_arrayref.

Output

Hashref, or undef when no row matches.

EXAMPLE

my $row = $join->fetchrow_hashref(entry => 'C001');
if (defined $row) {
    print "Name: $row->{name}, Tier: $row->{tier}\n";
} else {
    print "No record for C001\n";
}

# Positional: works when join_column is 'entry'
my $row2 = $join->fetchrow_hashref('C001');

count

SYNOPSIS

my $total  = $join->count();
my $active = $join->count(tier => 'gold');

DESCRIPTION

Returns the number of merged rows that satisfy the given criteria.

The full join is performed and the resulting rows are counted in Perl; no COUNT(*) is pushed down to the component databases.

API SPECIFICATION

Input

Same criteria syntax as selectall_arrayref.

Output

Non-negative integer.

EXAMPLE

my $total   = $join->count();
my $gold    = $join->count(tier => 'gold');
my $high    = $join->count(score => { '>' => 90 });

printf "%d total, %d gold-tier, %d high-scorers\n",
    $total, $gold, $high;

columns

SYNOPSIS

my $cols = $join->columns();

DESCRIPTION

Returns an arrayref of all column names visible in the merged view, deduplicated and sorted alphabetically.

The join_column appears exactly once, even if it exists under different local names in some databases (see join_map). Columns that have been hidden with remove_column or remove_columns do not appear.

The result is memoised: repeated calls are cheap.

API SPECIFICATION

Input

None.

Output

Arrayref of column name strings, sorted alphabetically.

EXAMPLE

my $cols = $join->columns();
print join(', ', @{$cols}), "\n";
# e.g. "entry, name, score, tier"

schema

SYNOPSIS

my $schema = $join->schema();

DESCRIPTION

Returns a merged schema hashref for all visible columns across all component databases. Each key is a column name; each value is the schema metadata hashref returned by Database::Abstraction::schema() for that column (typically { type, nullable, default, pk }).

When the same column name appears in more than one database the last database's metadata is used. Columns hidden with remove_column are not included.

The result is memoised.

API SPECIFICATION

Input

None.

Output

Hashref: column_name => { type => ..., nullable => ..., default => ..., pk => ... }.

EXAMPLE

my $schema = $join->schema();
for my $col (sort keys %{$schema}) {
    my $info = $schema->{$col};
    printf "%-15s type=%-10s nullable=%s\n",
        $col, $info->{type}, $info->{nullable} ? 'yes' : 'no';
}

updated

SYNOPSIS

my $ts = $join->updated();

DESCRIPTION

Returns the Unix timestamp of the most recent modification across all component databases. This is the maximum of all individual updated() return values.

Use this to implement simple cache-invalidation logic: if updated() has advanced since your last snapshot, re-query.

API SPECIFICATION

Input

None.

Output

Unix timestamp (positive integer).

EXAMPLE

my $last_modified = $join->updated();
if ($last_modified > $my_cache_timestamp) {
    $my_cache = $join->selectall_arrayref();
    $my_cache_timestamp = $last_modified;
}

set_logger

SYNOPSIS

$join->set_logger($log);

DESCRIPTION

Attaches a new logger object to the join and propagates it to every component database. The logger is used for diagnostic output by all component databases.

API SPECIFICATION

Input

$log    Positional: a logger object (required).
        Must support whatever interface Database::Abstraction expects.

Output

Returns C<$self> for method chaining.

EXAMPLE

# Log::Any is used here as an example; any object that implements
# debug() and info() (or whichever methods your component databases
# call internally) works equally well.
use Log::Any qw($log);

my $join = Database::Join->new(databases => [$db1, $db2], join_column => 'entry');
$join->set_logger($log);
# $log is now used by $join and by $db1 and $db2

add_database

SYNOPSIS

# Positional: database object as first argument
$join->add_database($db);

# Named: equivalent to the above
$join->add_database(database => $db);

# With options (mixed positional + named)
$join->add_database($db, remove_columns => ['internal_id']);
$join->add_database($db, join_column    => 'local_key_name');
$join->add_database($db, filter         => { score => { '>' => 60 } });

# Chainable
$join->add_database($db1)->add_database($db2, remove_columns => ['notes']);

DESCRIPTION

Adds one more Database::Abstraction subclass object to the logical view and immediately updates the column-ownership index.

After the call, all query methods return rows that include columns from the newly added database, and criteria on those new columns are routed to it automatically.

When a column name in the new database already exists in an earlier database, the new database becomes the authoritative source for that column (last-database-wins, the same rule that applies at construction time).

The join-column must be present in the new database (or declared via join_column). The logger is propagated to the new database if one is set.

add_database is the runtime equivalent of listing the database in the databases array to new. The optional join_column parameter is equivalent to a join_map entry; the optional filter parameter is equivalent to a filters entry.

API SPECIFICATION

Input

database       => { type => 'object',   required => 1 }
                  # A Database::Abstraction subclass instance.
                  #
                  # DOMAIN -- EP valid:   blessed object that passes
                  #                       isa('Database::Abstraction').
                  # DOMAIN -- EP invalid: non-reference, unblessed ref, wrong class,
                  #                       or non-reference non-key scalar (the guard at
                  #                       the top of add_database rejects it with
                  #                       error_invalid_db before validate_strict runs).

join_column    => { type => 'string',   optional => 1 }
                  # The name of the join key in THIS new database,
                  # when it differs from the canonical join_column.
                  #
                  # DOMAIN -- EP valid:   any string that exists as a column in the
                  #                       new database.
                  # DOMAIN -- EP invalid: string absent from the new database's columns()
                  #                       => croak error_join_col_missing.

filter         => { type => 'hashref',  optional => 1 }
                  # Permanent criteria for this database only.
                  # Same format as selectall_arrayref.
                  #
                  # DOMAIN -- EP valid:   hashref of criteria (may be {} for no-op).
                  # DOMAIN -- EP absent:  no permanent filter applied; all rows visible.
                  # DOMAIN -- Key-set:    a non-empty filter makes this DB an inner-join
                  #                       partner regardless of the outer join_type.

remove_columns => { type => 'arrayref', optional => 1 }
                  # Column names from this database to hide.
                  #
                  # DOMAIN -- EP valid:   arrayref of strings; non-existent columns silently
                  #                       ignored; empty [] is a safe no-op.
                  # DOMAIN -- EP invalid: join_column itself => croak error_remove_join_col.

Output

Returns C<$self> to support method chaining.

EXAMPLE

my $join = Database::Join->new(
    databases   => [ $customers ],
    join_column => 'entry',
);

# Add loyalty data; hide internal columns from it
$join->add_database($loyalty, remove_columns => ['audit_ts']);

# Add score data; only include rows with score > 60
$join->add_database($scores, filter => { score => { '>' => 60 } });

# Add a database whose join key has a different local name
$join->add_database($stnames, join_column => 'state_code');

# All three options combined, and chained
$join->add_database($db4,
    join_column    => 'ref_id',
    filter         => { active => 1 },
    remove_columns => ['legacy_col'],
);

PSEUDOCODE

determine the new database's index (length of current _dbs array)
extract the database object from positional or named argument
croak if it is not a Database::Abstraction subclass
register join_column alias in _join_map if different from canonical
register filter in _filters if provided
fetch column list from the new database
croak if the join key is missing from the new database
append the new database to _dbs and _db_cols
update _col_db: for each new column, point it at the new index
    (last-database-wins; skip removed columns and the local join alias)
invalidate _col_cache and _schema_cache
propagate logger if set
apply remove_columns if provided
return $self

MESSAGES

error_invalid_db       -- argument is not a Database::Abstraction subclass
error_join_col_missing -- join_column not found in the new database

remove_column

SYNOPSIS

$join->remove_column('email');

# Chainable
$join->remove_column('internal_id')->remove_column('audit_ts');

DESCRIPTION

Permanently hides a column from the merged view. After this call:

The join_column cannot be removed; attempting to do so will croak. Removing a column that does not exist in any database is silently ignored (the call is idempotent and safe). The columns() and schema() memoisation caches are cleared automatically.

API SPECIFICATION

Input

$col    Positional string: the column name to remove.

        DOMAIN -- EP valid:   any string; non-existent columns are silently
                              ignored (idempotent call, returns $self).
        DOMAIN -- EP invalid: join_column value => croak error_remove_join_col.
        DOMAIN -- BVA:        undef and '' are explicit no-ops (returns $self).
                              These are below the minimum meaningful string
                              length and are handled without any warning.

Output

Returns C<$self> to support method chaining.

EXAMPLE

# Hide private fields immediately after construction
my $join = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
)->remove_column('email')
 ->remove_column('internal_notes');

# Verify they are gone
my $cols = $join->columns();
# 'email' and 'internal_notes' are absent

MESSAGES

error_remove_join_col -- attempt to remove the join_column itself

query

Not supported. Database::Join does not implement the chained query builder. Calling this method will always croak with an explanatory message.

Use selectall_arrayref, selectall_array, fetchrow_hashref, or count instead.

execute

Not supported. Raw SQL cannot span heterogeneous backends that may use different database engines. Calling this method will always croak.

Use selectall_arrayref or fetchrow_hashref to query the joined view.

AUTOLOAD - column shortcut

Calling an unknown method whose name matches a visible column name performs a column lookup across the merged view.

SYNOPSIS

# Scalar context: value from the first matching row
my $name  = $join->name(entry => 'C001');

# List context: values from every matching row
my @tiers = $join->tier();

# With a positional join-key argument (when join_column is 'entry')
my $score = $join->score('C001');

DESCRIPTION

AUTOLOAD routes the call to the appropriate component database by looking up the column name in the internal column-ownership index.

When either join_map or filters is active, AUTOLOAD performs a full join query instead of delegating directly to the owning database. This is necessary because:

In list context, every matching merged row contributes one value to the returned list. In scalar context, only the first row's value is returned.

Calling a method whose name begins with _ (a private method) via AUTOLOAD will croak with a clear error message rather than being silently ignored.

EXAMPLE

# Lookup a single customer's name (scalar context)
my $name = $join->name('C001');   # 'C001' maps to entry => 'C001'
print "Name: $name\n";

# Get every tier value in the view (list context)
my @all_tiers = $join->tier();
my %freq;
$freq{$_}++ for @all_tiers;

# join_map active: AUTOLOAD runs a full join so the criteria are
# translated correctly between the canonical and local key names.
my @leesburg_states = sort $join->state('Leesburg');
# ['Florida', 'Virginia'] if Leesburg appears in two states

PSEUDOCODE

extract column name from $AUTOLOAD
return if DESTROY
croak if column name starts with '_' (private method guard)
croak if column name is not in _col_db (unknown column)
if join_map or filters are active:
    parse calling arguments using _parse_query_args
    call _joined_query to get all merged rows
    return map { $_->{col} } @rows  in list context
    return $rows[0]{col}            in scalar context
else:
    delegate directly to the owning database

ENCODING

All text that passes through Database::Join at the Perl layer (column names, criteria values, merged row values) is treated as opaque strings. Database::Join does not inspect, encode, or transform string content.

MESSAGES

The following messages can be produced by Database::Join. All messages can be localised by supplying an i18n object to new.

REPOSITORY

https://github.com/nigelhorne/Database-Join

SUPPORT

This module is provided as-is without any warranty.

SEE ALSO

SECURITY CONSIDERATIONS

Database::Join is a pure in-memory routing and merge layer. It never generates SQL strings, never opens files, and never calls system(), exec(), or eval(). The security properties described below are architectural guarantees, not run-time checks.

What Database::Join guarantees

What the caller is responsible for

API SPECIFICATION (security surface)

Input accepted by all query methods and passed through DJ to component databases:

Criterion values:
    type: scalar string | operator hashref { OP => scalar }
    validation: NONE (DJ trusts the caller; component DA is responsible)
    max size: unconstrained (OOM risk on very large values)

Column name keys in criteria:
    type: string
    validation: must be present in _col_db (else carp + drop)
    character set: any Perl string (including control chars); DJ does
                   not impose a character-set restriction on criteria KEYS

AUTOLOAD method-name-as-column:
    type: \w+ (enforced by Perl regex /::(\w+)$/)
    validation: must not start with '_'; must be in _col_db

FORMAL SPECIFICATION

Z calculus schemas for the key invariants and operations. Unicode is used throughout this section as required by Z notation.

─── Database_Join ─────────────────────────────────────────────────
dbs            : seq DATABASE_ABSTRACTION
join_col       : NAME
join_type      : {left, inner, outer}
join_map       : ℕ ⇸ NAME
filters        : ℕ ⇸ CRITERIA
col_db         : NAME ⇸ ℕ
removed        : ℙ NAME
backend        : {array, sqlite, auto}
max_array_rows : ℕ
tmpdir         : PATH
───────────────────────────────────────────────────────────────────
#dbs ≥ 1
dom join_map ⊆ 0 ‥ (#dbs - 1)
dom filters  ⊆ 0 ‥ (#dbs - 1)
dom col_db   = (⋃ { i : 0 ‥ #dbs-1 • ran((dbs i).columns) }) \ removed
join_col ∉ removed
∀ i : 0 ‥ #dbs-1 •
    local_jc(i) = if i ∈ dom join_map then join_map(i) else join_col
∀ i : 0 ‥ #dbs-1 •
    local_jc(i) ∈ ran((dbs i).columns)

─── Init ──────────────────────────────────────────────────────────
ΔDatabase_Join
dbs?           : seq DATABASE_ABSTRACTION
join_col?      : NAME
join_type?     : {left, inner, outer}
join_map?      : ℕ ⇸ NAME
filters?       : ℕ ⇸ CRITERIA
removed?       : ℙ NAME
backend?       : {array, sqlite, auto}   -- default auto
max_array_rows? : ℕ                      -- default 10000
tmpdir?        : PATH                    -- default File::Spec->tmpdir
───────────────────────────────────────────────────────────────────
#dbs? ≥ 1
dbs'           = dbs?
join_col'      = join_col?
join_type'     = join_type?
join_map'      = join_map?
filters'       = filters?
col_db'        = buildColIndex(dbs?, join_col?, join_map?)
removed'       = removed?
backend'       = backend?
max_array_rows' = max_array_rows?
tmpdir'        = tmpdir?

─── SelectAllArrayref ─────────────────────────────────────────────
ΞDatabase_Join        -- state unchanged
criteria? : CRITERIA
result!   : seq MERGED_ROW
───────────────────────────────────────────────────────────────────
∀ c : dom criteria? • c ∈ dom col_db ∪ {join_col}
result! = joinedQuery(criteria?)
result! is sorted ascending by join_col value

─── AddDatabase ───────────────────────────────────────────────────
ΔDatabase_Join
db?         : DATABASE_ABSTRACTION
local_jc?   : NAME   -- optional; defaults to join_col
filter?     : CRITERIA   -- optional
remove?     : ℙ NAME     -- optional
───────────────────────────────────────────────────────────────────
db?.isa('Database::Abstraction')
local_jc? ∈ ran(db?.columns)
dbs'      = dbs ^ ⟨db?⟩
col_db'   = col_db ⊕ { c ↦ #dbs | c ∈ ran(db?.columns) \ {local_jc?} \ removed }
filters'  = if filter? ≠ ∅ then filters ⊕ {#dbs ↦ filter?} else filters
join_map' = if local_jc? ≠ join_col
            then join_map ⊕ {#dbs ↦ local_jc?}
            else join_map
removed'  = removed ∪ remove?

─── RemoveColumn ──────────────────────────────────────────────────
ΔDatabase_Join
col? : NAME
───────────────────────────────────────────────────────────────────
col? ≠ join_col
removed'  = removed ∪ {col?}
col_db'   = col_db \ {col?}
join_map' = join_map
filters'  = filters
dbs'      = dbs

collision_prefix

─── CollisionPrefix ───────────────────────────────────────────────
collision_prefix : ℕ ⇸ STRING
dbs              : seq DATABASE_ABSTRACTION
col_db           : NAME ⇸ ℕ
───────────────────────────────────────────────────────────────────
-- Only secondary databases (index > 0) carry a meaningful prefix:
dom collision_prefix ⊆ 1 ‥ (#dbs - 1)

-- Published name for column col from database i:
published(i, col) ==
    if i ∈ dom collision_prefix ∧ col ∈ dom col_db ∧ col_db(col) < i
    then (collision_prefix i) ^ "." ^ col
    else col

-- col_db routes the published name to the owning database:
col_db(published(i, col)) = i

-- The original column name is never removed from an earlier database:
∀ i : 1 ‥ #dbs-1; col : columns(dbs i) •
    published(i, col) ≠ col  ⟹
        ∃ j : 0 ‥ i-1 • col ∈ dom col_db ∧ col_db(col) = j

join_map

─── JoinMap ───────────────────────────────────────────────────────
join_map : ℕ ⇸ NAME
dbs      : seq DATABASE_ABSTRACTION
join_col : NAME
───────────────────────────────────────────────────────────────────
dom join_map ⊆ 0 ‥ (#dbs - 1)
∀ i : dom join_map • (join_map i) ∈ ran(dbs i).columns
∀ i : 0 ‥ (#dbs - 1) \ dom join_map •
    join_col ∈ ran(dbs i).columns

-- Resolution of the local join-key name for database i:
local_jc(i) == if i ∈ dom join_map then join_map(i) else join_col

-- The canonical name is always join_col; local_jc is never exposed.

SECURITY INVARIANTS

─── PartitionIsolation ─────────────────────────────────────────────
-- For every query call with criteria C and column col ≠ join_col:
∀ i : 0 ‥ #dbs-1 •
    i ≠ _col_db(col)  ⟹  col ∉ dom(per_db(i))

-- Unknown column is dropped before any database sees it:
col ∉ dom(_col_db) ∧ col ≠ join_col  ⟹
    (∀ i : 0 ‥ #dbs-1 • col ∉ dom(per_db(i)))

─── NoCodeExecution ────────────────────────────────────────────────
-- DJ contains no call to system(), exec(), open(PIPE), or eval().
-- Hostile criterion values therefore cannot achieve code execution
-- within the Database::Join layer.
∀ v : VALUE • _joined_query({col ↦ v}) ≠ ⊥ due to code injection

filters

─── Filters ─────────────────────────────────────────────────────
filters  : ℕ ⇸ CRITERIA
dbs      : seq DATABASE_ABSTRACTION
─────────────────────────────────────────────────────────────────
dom filters ⊆ 0 ‥ (#dbs - 1)

-- A filtered database i always contributes to key-set intersection.
-- For each query with criteria C:
effective_criteria(i, C) ==
    if i ∈ dom filters
    then merge_criteria(filters(i), partition(C, i))
    else partition(C, i)

-- Criteria merging (AND semantics for operator hashrefs):
merge_criteria(base, extra) ==
    { col : dom base ∪ dom extra •
        if col ∈ dom base ∩ dom extra
           ∧ base(col) ∈ HASHREF ∧ extra(col) ∈ HASHREF
        then col ↦ base(col) ∪ extra(col)   -- operator union
        else col ↦ (if col ∈ dom extra then extra(col) else base(col)) }

selectall_arrayref

selectall_arrayref : CRITERIA → seq MERGED_ROW
pre:  ∀ col : dom criteria • col ∈ dom self._col_db ∪ {self._join_col}
post: result = _joined_query(criteria)
      result is sorted ascending by join_col value

selectall_array

selectall_array : CRITERIA → seq MERGED_ROW | MERGED_ROW?
pre:  same as selectall_arrayref
post: wantarray  => result = @{ selectall_arrayref(criteria) }
      !wantarray => result = selectall_arrayref(criteria)[0]  (or undef)

fetchrow_hashref

fetchrow_hashref : CRITERIA → MERGED_ROW?
post: result = selectall_arrayref(criteria)[0]  (or undef if empty)

count

count : CRITERIA → ℕ
post: result = #selectall_arrayref(criteria)

columns

columns : → seq NAME
post: result = sort(
          (⋃ { i : 0 ‥ #dbs-1 • ran(dbs(i).columns) }
           \ dom removed_cols
           \ { local_jc(i) | i ∈ dom join_map ∧ local_jc(i) ≠ join_col })
      )

schema

schema : → NAME ⇸ SCHEMA_INFO
post: dom(result) = ran(columns())
      ∀ col : dom(result) •
          result(col) = (last database containing col).schema()(col)

updated

updated : → ℕ
post: result = max { i : 0 ‥ #dbs-1 • dbs(i).updated() }

remove_column

remove_column : NAME → Database_Join
pre:  col ≠ self._join_col
post: self'._removed_cols = self._removed_cols ∪ {col}
      self'._col_db       = self._col_db \ {col}
      self'._col_cache    = undef
      self'._schema_cache = undef

AUTOLOAD

AUTOLOAD : NAME × CRITERIA → VALUE | seq VALUE
pre:  col ∈ dom self._col_db
      col does not begin with '_'
post: let rows = _joined_query(criteria)
      wantarray  => result = { r : rows • r(col) }
      !wantarray => result = rows(0)(col)  (or undef if rows is empty)

backend

─── BackendDispatch ───────────────────────────────────────────────
backend        : {array, sqlite, auto}
max_array_rows : ℕ
───────────────────────────────────────────────────────────────────

-- row_count(db): cheaply count rows in a component database.
-- Uses dbi_source() COUNT(*) SQL for SQLite-backed sources,
-- or the DA's own count() method when defined in its own package.
-- Returns ⊥ (bottom / unknown) when neither is available.
row_count(db) ==
    if (db has dbi_source() returning a SQLite dbh)
    then SELECT COUNT(*) FROM source_table
    else if (defined &{ref(db) ^ "::count"})
    then db.count()
    else ⊥

-- Combined row count across all component databases.
-- If any database returns ⊥, total is ⊥ (cannot determine).
total_count ==
    if ∀ i : 0 ‥ #dbs-1 • row_count(dbs i) ≠ ⊥
    then Σ { i : 0 ‥ #dbs-1 • row_count(dbs i) }
    else ⊥

-- Dispatch rule for _joined_query:
use_sqlite(C) ==
    backend = 'sqlite'
    ∨ (backend = 'auto' ∧ total_count ≠ ⊥ ∧ total_count > max_array_rows)

_joined_query(C) ==
    if use_sqlite(C)
    then _sqlite_join(C)
    else _joined_query_array(C)

-- Result identity invariant: both paths return identical rows.
∀ C : CRITERIA •
    _sqlite_join(C) = _joined_query_array(C)

STATE DIAGRAM

Database::Join objects follow three independent finite state machines (FSMs). Each FSM is described with an ASCII diagram showing valid states (boxes), the triggers that cause transitions (arrows), and important side-effects.

FSM 1: Object Lifecycle

Governs the structural state of a Database::Join instance. Query methods (selectall_arrayref, fetchrow_hashref, count, columns, schema, updated) are schema-preserving (Xi-transitions) and are not shown because they do not change state.

[pre-creation]
     |
     | new( databases => [...], join_column => '...' )
     |   Side-effect: _col_db routing table built;
     |                _autoload_pk cached from dbs[0]{id}
     v
[CONSTRUCTED] <-----------------------------------------+
     |    |                                              |
     |    +--------------------------------------------+ |
     |    (query methods: no structural change)         | |
     |                                                  | |
     |-- remove_column( col ) -------> [COL_REMOVED] <--+ |
     |                                      |    |        |
     |   Side-effect: col removed from       |    |        |
     |   _col_db; _col_cache and             +----+        |
     |   _schema_cache cleared.              (idempotent;  |
     |   Join column cannot be removed.)      chainable)   |
     |                                                     |
     +-- add_database( db ) ----------> [DB_ADDED] <------+
                                            |    |
         Side-effect: new columns added;    |    | add_database( db )
         _col_db extended; SQLite cache     |    | (chainable; each
         invalidated (if any).              +----+  extends the view)

Note: COL_REMOVED and DB_ADDED are not mutually exclusive.
Both transitions are legal on any valid object, in any order.

Illegal triggers (always croak; object state is not changed):

  Trigger                              Error
  -----------------------------------  ---------------------------------
  new( databases => [] )               error_no_databases
  remove_column( join_column )         error_remove_join_column
  add_database( non-reference )        error_invalid_database
  new() with join_col absent from DB   error_join_col_absent

FSM 2: SQLite Cache Lifecycle

Governs the temporary SQLite cache used by the backend='sqlite' and backend='auto' join paths. The cache does not exist until the first query on the SQLite path.

[ABSENT] <----- add_database( db )
   |                  |
   |  (no temp file)  | Side-effect: old DBI handle disconnected;
   |                  |   _sqlite_cache deleted.
   |                  |
   |                  +<-------------------------------------------+
   |                                                               |
   | first query on SQLite path                                    |
   | Side-effect: File::Temp db created in tmpdir;                 |
   |   DBI connected; sources ATTACHed or spilled;                 |
   |   _sqlite_cache = { dbh, tmpfile, n, updated, ... }           |
   v                                                               |
[FRESH] <--+                                                       |
   |        |                                                      |
   |        | subsequent queries                                   |
   |        | (cache reused; refaddr of _sqlite_cache unchanged)   |
   +--------+                                                      |
   |                                                               |
   | updated() timestamp of any source DA changes                  |
   | -- OR -- source row count changes                             |
   | Side-effect: none yet (_cache_fresh returns false)            |
   v                                                               |
[STALE]                                                            |
   |                                                               |
   | next query                                                    |
   | Side-effect: old DBI handle disconnected; old temp file       |
   |   unlinked; new temp file built from current source data.     |
   +---------------------------------------------------------------+
   (transitions to FRESH)

On object DESTROY:
  FRESH/STALE:  DBI handle disconnected; File::Temp object released
                (temp file unlinked by File::Temp DESTROY).
  ABSENT:       No temp file exists; no-op.

FSM 3: Column Visibility (per column)

Each column in the logical view independently follows a two-state machine. Transition from VISIBLE to REMOVED is one-way: add_database never restores a column that is in _removed_cols.

[VISIBLE] <-- initial state for every column at construction
     |    |
     |    | query / columns() / schema()
     |    | (column present in results; no state change)
     +----+
     |
     | remove_column( col )
     | Side-effect: col deleted from _col_db;
     |   _col_cache and _schema_cache cleared.
     v
[REMOVED] <---+
     |         |
     |         | remove_column( col ) again
     |         | (idempotent; no error; no second side-effect)
     +---------+

One-way invariant:
  If col is in _removed_cols, then add_database( db_that_has_col )
  does NOT re-add it.  Formal: col_db' = col_db ⊕ { c | c in
  ran(db.columns) \ {local_jc} \ removed }.

Illegal trigger:
  remove_column( join_column )  -- error_remove_join_column (croaks;
                                   state unchanged)

AUTHOR

Nigel Horne, <njh@nigelhorne.com>

LICENSE AND COPYRIGHT

Copyright (C) 2026 Nigel Horne.

Usage is subject to the GPL2 licence terms. If you use it, please let me know.