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515b9ad05a
MDEV-28073 Slow query performance in MariaDB when using many table The idea is to prefer and chain EQ_REF tables (tables that uses an unique key to find a row) when searching for the best table combination. This significantly reduces row combinations that has to be examined. This is optimization is enabled when setting optimizer_prune_level=2 (which is now default). Implementation: - optimizer_prune_level has a new level, 2, which enables EQ_REF optimization in addition to the pruning done by level 1. Level 2 is now default. - Added JOIN::eq_ref_tables that contains bits of tables that could use potentially use EQ_REF access in the query. This is calculated in sort_and_filter_keyuse() Under optimizer_prune_level=2: - When the greedy_optimizer notices that the preceding table was an EQ_REF table, it tries to add an EQ_REF table next. If an EQ_REF table exists, only this one will be considered at this level. We also collect all EQ_REF tables chained by the next levels and these are ignored on the starting level as we have already examined these. If no EQ_REF table exists, we continue as normal. This optimization speeds up the greedy_optimizer combination test with ~25% Other things: - I ported the changes in MySQL 5.7 to greedy_optimizer.test to MariaDB to be able to ensure we can handle all cases that MySQL can do. - I have run all tests with --mysqld=--optimizer_prune_level=1 to verify that there where no test changes.
57 lines
1.6 KiB
PHP
57 lines
1.6 KiB
PHP
# include/check_qep.inc
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#
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# SUMMARY
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#
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# Designed to be used together with include/expect_qep.inc
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#
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# $query should be assigned a select statement using
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# straight_join to force the tables to be joined in most
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# optimal order.
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#
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# expect_qep.inc will then store the estimated 'Last_query_cost'
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# and total # 'Handler_read%' for this straight_joined query.
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#
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# We should then assign a non-straight_join'ed version of
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# the same query to $query and execute it using
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# 'include/check_qep.inc'. Its estimated cost and
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# #handler_reads will then be verified against the
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# previous straight_joined query.
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#
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# USAGE
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#
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# let $query= <select straight_join optimal statement>;
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# --source include/expect_qep.inc
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# let $query= <select statement>;
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# --source include/check_qep.inc
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#
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# EXAMPLE
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# t/greedy_optimizer.test
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#
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flush status;
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eval EXPLAIN $query;
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eval $query;
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let $cost=
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query_get_value(SHOW STATUS LIKE 'Last_query_cost', Value, 1);
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--disable_warnings
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let $reads=
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`select sum(variable_value)
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from information_schema.session_status
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where VARIABLE_NAME like 'Handler_read%'`;
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--enable_warnings
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#echo Cost: $cost, Handler_reads: $reads;
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if ($cost != $best_cost)
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{ echo ### FAILED: Query_cost: $cost, expected: $best_cost ###;
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}
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# Difference in handler reads are ok as tables in MariaDB are sorted according
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# to order in the query and the tables in greedy_optimizer.inc has reference to
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# rows that does not exists, so different table orders will do different
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# number of reads
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if ($reads != $best_reads)
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{ echo ### NOTE: Handler_reads: $reads, expected: $best_reads ###;
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}
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