DBIx::Class::SQLMaker::LimitDialects(3p)

NAME
DBIx::Class::SQLMaker::LimitDialects - SQL::Abstract::Limit-like functionality in DBIx::Class::SQLMaker

DESCRIPTION
DBIC's SQLMaker stack replicates and surpasses all 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 ...

Supported 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 AS rownum__index FROM (
   SELECT ...
  ) WHERE ROWNUM <= ($limit+$offset)
 ) WHERE rownum__index >= ($offset+1)

or

 SELECT * FROM (
  SELECT *, ROWNUM AS 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.

FURTHER QUESTIONS?
Check the list of additional DBIC resources.

COPYRIGHT AND LICENSE
This module is free software copyright by the DBIx::Class (DBIC) authors. You can redistribute it and/or modify it under the same terms as the DBIx::Class library.