From da5214831d3d0f0880b4068a06ce062ca703293f Mon Sep 17 00:00:00 2001 From: unknown Date: Thu, 17 May 2012 13:46:05 +0300 Subject: Fix for bug lp:944706, task MDEV-193 The patch enables back constant subquery execution during query optimization after it was disabled during the development of MWL#89 (cost-based choice of IN-TO-EXISTS vs MATERIALIZATION). The main idea is that constant subqueries are allowed to be executed during optimization if their execution is not expensive. The approach is as follows: - Constant subqueries are recursively optimized in the beginning of JOIN::optimize of the outer query. This is done by the new method JOIN::optimize_constant_subqueries(). This is done so that the cost of executing these queries can be estimated. - Optimization of the outer query proceeds normally. During this phase the optimizer may request execution of non-expensive constant subqueries. Each place where the optimizer may potentially execute an expensive expression is guarded with the predicate Item::is_expensive(). - The implementation of Item_subselect::is_expensive has been extended to use the number of examined rows (estimated by the optimizer) as a way to determine whether the subquery is expensive or not. - The new system variable "expensive_subquery_limit" controls how many examined rows are considered to be not expensive. The default is 100. In addition, multiple changes were needed to make this solution work in the light of the changes made by MWL#89. These changes were needed to fix various crashes and wrong results, and legacy bugs discovered during development. --- mysql-test/r/key.result | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'mysql-test/r/key.result') diff --git a/mysql-test/r/key.result b/mysql-test/r/key.result index 4035f61e36b..e215caadb1c 100644 --- a/mysql-test/r/key.result +++ b/mysql-test/r/key.result @@ -598,8 +598,8 @@ VALUES EXPLAIN SELECT 1 FROM t1 AS t1_outer WHERE (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2) > 12; id select_type table type possible_keys key key_len ref rows Extra -1 PRIMARY t1_outer index NULL a 10 NULL 15 Using index -2 SUBQUERY t1 range NULL a 5 NULL 8 Using index for group-by +1 PRIMARY NULL NULL NULL NULL NULL NULL NULL Impossible WHERE +2 SUBQUERY t1 index NULL a 10 NULL 15 Using index SELECT 1 as RES FROM t1 AS t1_outer WHERE (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2) > 12; RES -- cgit v1.2.1