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.IX Title "DBIx::Class::SQLMaker::LimitDialects 3pm"
.TH DBIx::Class::SQLMaker::LimitDialects 3pm "2022-05-21" "perl v5.34.0" "User Contributed Perl Documentation"
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.nh
.SH "NAME"
DBIx::Class::SQLMaker::LimitDialects \- SQL::Abstract::Limit\-like functionality in DBIx::Class::SQLMaker
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\s-1DBIC\s0'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.
.SH "SQL LIMIT DIALECTS"
.IX Header "SQL LIMIT DIALECTS"
Note that the actual implementations listed below never use \f(CW\*(C`*\*(C'\fR 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.
.PP
Currently the provided dialects are:
.SS "LimitOffset"
.IX Subsection "LimitOffset"
.Vb 1
\& SELECT ... LIMIT $limit OFFSET $offset
.Ve
.PP
Supported by \fBPostgreSQL\fR and \fBSQLite\fR
.SS "LimitXY"
.IX Subsection "LimitXY"
.Vb 1
\& SELECT ... LIMIT $offset, $limit
.Ve
.PP
Supported by \fBMySQL\fR and any SQL::Statement based \s-1DBD\s0
.SS "RowNumberOver"
.IX Subsection "RowNumberOver"
.Vb 5
\& SELECT * FROM (
\&  SELECT *, ROW_NUMBER() OVER( ORDER BY ... ) AS RNO_\|_ROW_\|_INDEX FROM (
\&   SELECT ...
\&  )
\& ) WHERE RNO_\|_ROW_\|_INDEX BETWEEN ($offset+1) AND ($limit+$offset)
.Ve
.PP
\&\s-1ANSI\s0 standard Limit/Offset implementation. Supported by \fB\s-1DB2\s0\fR and
\&\fB\s-1MSSQL\s0 >= 2005\fR.
.SS "SkipFirst"
.IX Subsection "SkipFirst"
.Vb 1
\& SELECT SKIP $offset FIRST $limit * FROM ...
.Ve
.PP
Supported by \fBInformix\fR, almost like LimitOffset. According to
SQL::Abstract::Limit \f(CW\*(C`... SKIP $offset LIMIT $limit ...\*(C'\fR is also supported.
.SS "FirstSkip"
.IX Subsection "FirstSkip"
.Vb 1
\& SELECT FIRST $limit SKIP $offset * FROM ...
.Ve
.PP
Supported by \fBFirebird/Interbase\fR, reverse of SkipFirst. According to
SQL::Abstract::Limit \f(CW\*(C`... ROWS $limit TO $offset ...\*(C'\fR is also supported.
.SS "RowNum"
.IX Subsection "RowNum"
Depending on the resultset attributes one of:
.PP
.Vb 5
\& SELECT * FROM (
\&  SELECT *, ROWNUM AS rownum_\|_index FROM (
\&   SELECT ...
\&  ) WHERE ROWNUM <= ($limit+$offset)
\& ) WHERE rownum_\|_index >= ($offset+1)
.Ve
.PP
or
.PP
.Vb 5
\& SELECT * FROM (
\&  SELECT *, ROWNUM AS rownum_\|_index FROM (
\&    SELECT ...
\&  )
\& ) WHERE rownum_\|_index BETWEEN ($offset+1) AND ($limit+$offset)
.Ve
.PP
or
.PP
.Vb 3
\& SELECT * FROM (
\&    SELECT ...
\&  ) WHERE ROWNUM <= ($limit+1)
.Ve
.PP
Supported by \fBOracle\fR.
.SS "Top"
.IX Subsection "Top"
.Vb 1
\& SELECT * FROM
\&
\& SELECT TOP $limit FROM (
\&  SELECT TOP $limit FROM (
\&   SELECT TOP ($limit+$offset) ...
\&  ) ORDER BY $reversed_original_order
\& ) ORDER BY $original_order
.Ve
.PP
Unreliable Top-based implementation, supported by \fB\s-1MSSQL\s0 < 2005\fR.
.PP
\fI\s-1CAVEAT\s0\fR
.IX Subsection "CAVEAT"
.PP
Due to its implementation, this limit dialect returns \fBincorrect results\fR
when \f(CW$limit\fR+$offset > total amount of rows in the resultset.
.SS "FetchFirst"
.IX Subsection "FetchFirst"
.Vb 10
\& SELECT * FROM
\& (
\& SELECT * FROM (
\&  SELECT * FROM (
\&   SELECT * FROM ...
\&  ) ORDER BY $reversed_original_order
\&    FETCH FIRST $limit ROWS ONLY
\& ) ORDER BY $original_order
\&   FETCH FIRST $limit ROWS ONLY
\& )
.Ve
.PP
Unreliable FetchFirst-based implementation, supported by \fB\s-1IBM DB2\s0 <= V5R3\fR.
.PP
\fI\s-1CAVEAT\s0\fR
.IX Subsection "CAVEAT"
.PP
Due to its implementation, this limit dialect returns \fBincorrect results\fR
when \f(CW$limit\fR+$offset > total amount of rows in the resultset.
.SS "GenericSubQ"
.IX Subsection "GenericSubQ"
.Vb 6
\& SELECT * FROM (
\&  SELECT ...
\& )
\& WHERE (
\&  SELECT COUNT(*) FROM $original_table cnt WHERE cnt.id < $original_table.id
\& ) BETWEEN $offset AND ($offset+$rows\-1)
.Ve
.PP
This is the most evil limit \*(L"dialect\*(R" (more of a hack) for \fIreally\fR 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 \*(L"RowNum\*(R"\-like
index). Of course this implies the set can only be ordered by a single unique
column.
.PP
Also note that this technique can be and often is \fBexcruciatingly slow\fR. You
may have much better luck using \*(L"software_limit\*(R" in DBIx::Class::ResultSet
instead.
.PP
Currently used by \fBSybase \s-1ASE\s0\fR, due to lack of any other option.
.SH "FURTHER QUESTIONS?"
.IX Header "FURTHER QUESTIONS?"
Check the list of additional \s-1DBIC\s0 resources.
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This module is free software copyright
by the DBIx::Class (\s-1DBIC\s0) authors. You can
redistribute it and/or modify it under the same terms as the
DBIx::Class library.