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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.
16 lines
820 B
Plaintext
16 lines
820 B
Plaintext
create table t1 (test_name text);
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create table t2 (variable_name text);
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load data infile "MYSQLTEST_VARDIR/tmp/sys_vars.all_vars.txt" into table t1;
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insert into t2 select variable_name from information_schema.global_variables;
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insert into t2 select variable_name from information_schema.session_variables;
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update t2 set variable_name= replace(variable_name, "PERFORMANCE_SCHEMA_", "PFS_");
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select distinct variable_name as `there should be *no* long test name listed below:` from t2
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where length(variable_name) > 50;
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there should be *no* long test name listed below:
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select distinct variable_name as `there should be *no* variables listed below:` from t2
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left join t1 on variable_name=test_name where test_name is null;
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there should be *no* variables listed below:
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expensive_subquery_limit
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drop table t1;
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drop table t2;
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