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Fix for bug lp:992405

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The patch backports two patches from mysql 5.6:
- BUG#12640437: USING SQL_BUFFER_RESULT RESULTS IN A DIFFERENT QUERY OUTPUT
- Bug#12578908: SELECT SQL_BUFFER_RESULT OUTPUTS TOO MANY ROWS WHEN GROUP IS OPTIMIZED AWAY

Original comment:
-----------------
3714 Jorgen Loland	2012-03-01
      BUG#12640437 - USING SQL_BUFFER_RESULT RESULTS IN A DIFFERENT 
                     QUERY OUTPUT
      
      For all but simple grouped queries, temporary tables are used to
      resolve grouping. In these cases, the list of grouping fields is
      stored in the temporary table and grouping is resolved
      there (e.g. by adding a unique constraint on the involved
      fields). Because of this, grouping is already done when the rows
      are read from the temporary table.
      
      In the case where a group clause may be optimized away, grouping
      does not have to be resolved using a temporary table. However, if
      a temporary table is explicitly requested (e.g. because the
      SQL_BUFFER_RESULT hint is used, or the statement is
      INSERT...SELECT), a temporary table is used anyway. In this case,
      the temporary table is created with an empty group list (because
      the group clause was optimized away) and it will therefore not
      create groups. Since the temporary table does not take care of
      grouping, JOIN::group shall not be set to false in 
      make_simple_join(). This was fixed in bug 12578908. 
      
      However, there is an exception where make_simple_join() should
      set JOIN::group to false even if the query uses a temporary table
      that was explicitly requested but is not strictly needed. That
      exception is if the loose index scan access method (explain
      says "Using index for group-by") is used to read into the 
      temporary table. With loose index scan, grouping is resolved 
      by the access method. This is exactly what happens in this bug.
This commit is contained in:
unknown
2012-05-07 11:02:58 +03:00
parent c9a73aa204
commit 213476ef3e
3 changed files with 132 additions and 3 deletions
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27
sql/sql_select.cc
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@@ -1306,7 +1306,6 @@ JOIN::optimize()
simple_order=1;
select_distinct= 0; // No need in distinct for 1 row
group_optimized_away= 1;
implicit_grouping= TRUE;
}
calc_group_buffer(this, group_list);
@@ -6072,7 +6071,31 @@ JOIN::make_simple_join(JOIN *parent, TABLE *temp_table)
tmp_table_param.copy_field= tmp_table_param.copy_field_end=0;
first_record= sort_and_group=0;
send_records= (ha_rows) 0;
group= 0;
if (group_optimized_away && !tmp_table_param.precomputed_group_by)
{
/*
If grouping has been optimized away, a temporary table is
normally not needed unless we're explicitly requested to create
one (e.g. due to a SQL_BUFFER_RESULT hint or INSERT ... SELECT).
In this case (grouping was optimized away), temp_table was
created without a grouping expression and JOIN::exec() will not
perform the necessary grouping (by the use of end_send_group()
or end_write_group()) if JOIN::group is set to false.
There is one exception: if the loose index scan access method is
used to read into the temporary table, grouping and aggregate
functions are handled.
*/
// the temporary table was explicitly requested
DBUG_ASSERT(test(select_options & OPTION_BUFFER_RESULT));
// the temporary table does not have a grouping expression
DBUG_ASSERT(!temp_table->group);
}
else
group= false;
row_limit= unit->select_limit_cnt;
do_send_rows= row_limit ? 1 : 0;