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DBIx::Class::SQLMaker::LimitDialects(3)er Contributed Perl Documentationx::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 limitdialect
       dialect provider.

SQL LIMIT DIALECTS
       Note that the actual implementations listed below never use "*" literally.  Instead proper re-aliasing realiasing
       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.

   RowCountOrGenericSubQ
       This is not exactly a limit dialect, but more of a proxy for Sybase ASE.  If no $offset is supplied
       the limit is simply performed as:

        SET ROWCOUNT $limit
        SELECT ...
        SET ROWCOUNT 0

       Otherwise we fall back to "GenericSubQ"

   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 v5.16.2                                     2012-08-23          DBIx::Class::SQLMaker::LimitDialects(3)