DBIx::Class::SQLMaker::LimitDialects(3) User Contributed Perl Documentation DBIx::Class::SQLMaker::LimitDialects(3)
NAME
DBIx::Class::SQLMaker::LimitDialects - SQL::Abstract::Limit-like functionality for DBIx::Class::SQLMaker
DESCRIPTION
This module replicates a lot of the functionality originally found in SQL::Abstract::Limit. While simple limits would work as-is, the more
complex dialects that require e.g. subqueries could not be reliably implemented without taking full advantage of the metadata locked within
DBIx::Class::ResultSource classes. After reimplementation of close to 80% of the SQL::Abstract::Limit functionality it was deemed more
practical to simply make an independent DBIx::Class-specific limit-dialect provider.
SQL LIMIT DIALECTS
Note that the actual implementations listed below never use "*" literally. Instead proper re-aliasing of selectors and order criteria is
done, so that the limit dialect are safe to use on joined resultsets with clashing column names.
Currently the provided dialects are:
LimitOffset
SELECT ... LIMIT $limit OFFSET $offset
Supported by PostgreSQL and SQLite
LimitXY
SELECT ... LIMIT $offset $limit
Supported by MySQL and any SQL::Statement based DBD
RowNumberOver
SELECT * FROM (
SELECT *, ROW_NUMBER() OVER( ORDER BY ... ) AS RNO__ROW__INDEX FROM (
SELECT ...
)
) WHERE RNO__ROW__INDEX BETWEEN ($offset+1) AND ($limit+$offset)
ANSI standard Limit/Offset implementation. Supported by DB2 and MSSQL >= 2005.
SkipFirst
SELECT SKIP $offset FIRST $limit * FROM ...
Suported by Informix, almost like LimitOffset. According to SQL::Abstract::Limit "... SKIP $offset LIMIT $limit ..." is also supported.
FirstSkip
SELECT FIRST $limit SKIP $offset * FROM ...
Supported by Firebird/Interbase, reverse of SkipFirst. According to SQL::Abstract::Limit "... ROWS $limit TO $offset ..." is also
supported.
RowNum
Depending on the resultset attributes one of:
SELECT * FROM (
SELECT *, ROWNUM rownum__index FROM (
SELECT ...
) WHERE ROWNUM <= ($limit+$offset)
) WHERE rownum__index >= ($offset+1)
or
SELECT * FROM (
SELECT *, ROWNUM rownum__index FROM (
SELECT ...
)
) WHERE rownum__index BETWEEN ($offset+1) AND ($limit+$offset)
or
SELECT * FROM (
SELECT ...
) WHERE ROWNUM <= ($limit+1)
Supported by Oracle.
Top
SELECT * FROM
SELECT TOP $limit FROM (
SELECT TOP $limit FROM (
SELECT TOP ($limit+$offset) ...
) ORDER BY $reversed_original_order
) ORDER BY $original_order
Unreliable Top-based implementation, supported by MSSQL < 2005.
CAVEAT
Due to its implementation, this limit dialect returns incorrect results when $limit+$offset > total amount of rows in the resultset.
FetchFirst
SELECT * FROM
(
SELECT * FROM (
SELECT * FROM (
SELECT * FROM ...
) ORDER BY $reversed_original_order
FETCH FIRST $limit ROWS ONLY
) ORDER BY $original_order
FETCH FIRST $limit ROWS ONLY
)
Unreliable FetchFirst-based implementation, supported by IBM DB2 <= V5R3.
CAVEAT
Due to its implementation, this limit dialect returns incorrect results when $limit+$offset > total amount of rows in the resultset.
GenericSubQ
SELECT * FROM (
SELECT ...
)
WHERE (
SELECT COUNT(*) FROM $original_table cnt WHERE cnt.id < $original_table.id
) BETWEEN $offset AND ($offset+$rows-1)
This is the most evil limit "dialect" (more of a hack) for really stupid databases. It works by ordering the set by some unique column, and
calculating the amount of rows that have a less-er value (thus emulating a "RowNum"-like index). Of course this implies the set can only be
ordered by a single unique column.
Also note that this technique can be and often is excruciatingly slow. You may have much better luck using "software_limit" in
DBIx::Class::ResultSet instead.
Currently used by Sybase ASE, due to lack of any other option.
AUTHORS
See "CONTRIBUTORS" in DBIx::Class.
LICENSE
You may distribute this code under the same terms as Perl itself.
perl v5.18.2 2014-01-22 DBIx::Class::SQLMaker::LimitDialects(3)