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Manual Reference Pages  -  DBIX::CLASS::SQLMAKER::LIMITDIALECTS (3)

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NAME

DBIx::Class::SQLMaker::LimitDialects - SQL::Abstract::Limit-like functionality for DBIx::Class::SQLMaker

CONTENTS

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 <B>PostgreSQLB> and <B>SQLiteB>

    LimitXY



 SELECT ... LIMIT $offset, $limit



Supported by <B>MySQLB> 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 <B>DB2B> and <B>MSSQL >= 2005B>.

    SkipFirst



 SELECT SKIP $offset FIRST $limit * FROM ...



Suported by <B>InformixB>, almost like LimitOffset. According to SQL::Abstract::Limit ... SKIP $offset LIMIT $limit ... is also supported.

    FirstSkip



 SELECT FIRST $limit SKIP $offset * FROM ...



Supported by <B>Firebird/InterbaseB>, 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 <B>OracleB>.

    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 <B>MSSQL < 2005B>.

CAVEAT

Due to its implementation, this limit dialect returns <B>incorrect resultsB> 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 <B>IBM DB2 <= V5R3B>.

CAVEAT

Due to its implementation, this limit dialect returns <B>incorrect resultsB> 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 <B>excruciatingly slowB>. You may have much better luck using software_limit in DBIx::Class::ResultSet instead.

Currently used by <B>Sybase ASEB>, 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.
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perl v5.20.3 DBIX::CLASS::SQLMAKER::LIMITDIALECTS (3) 2016-02-11

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