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Change cost for REF to take into account cost for 1 extra key read_next
The main difference in code path between EQ_REF and REF is that for REF we have to do an extra read_next on the index to check that there is no more matching rows. Before this patch we added a preference of EQ_REF by ensuring that REF would always estimate to find at least 2 rows. This patch adds the cost of the extra key read_next to REF access and removes the code that limited REF to at least 2 rows. For some queries this can have a big effect as the total estimated rows will be halved for each REF table with 1 rows. multi_range cost calculations are also changed to take into account the difference between EQ_REF and REF. The effect of the patch to the test suite: - About 80 test case changed - Almost all changes where for EXPLAIN where estimated rows for REF where changed from 2 to 1. - A few test cases using explain extended had a change of 'filtered'. This is because of the estimated rows are now closer to the calculated selectivity. - A very few test had a change of table order. This is because the change of estimated rows from 2 to 1 or the small cost change for REF (main.subselect_sj_jcl6, main.group_by, main.dervied_cond_pushdown, main.distinct, main.join_nested, main.order_by, main.join_cache) - No key statistics and the estimated rows are now smaller which cased estimated filtering to be lower. (main.subselect_sj_mat) - The number of total rows are halved. (main.derived_cond_pushdown) - Plans with 1 row changed to use RANGE instead of REF. (main.group_min_max) - ALL changed to REF (main.key_diff) - Key changed from ref + index_only to PRIMARY key for InnoDB, as OPTIMIZER_ROW_LOOKUP_COST + OPTIMIZER_ROW_NEXT_FIND_COST is smaller than OPTIMIZER_KEY_LOOKUP_COST + OPTIMIZER_KEY_NEXT_FIND_COST. (main.join_outer_innodb) - Cost changes printouts (main.opt_trace*) - Result order change (innodb_gis.rtree)
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Monty
@ -345,7 +345,7 @@ patient_uq clinic_uq
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explain extended select * from t6 where exists (select * from t7 where uq = clinic_uq);
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id select_type table type possible_keys key key_len ref rows filtered Extra
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1 PRIMARY t7 index PRIMARY PRIMARY 4 NULL 2 100.00 Using index
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1 PRIMARY t6 ref i1 i1 5 test.t7.uq 2 100.00
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1 PRIMARY t6 ref i1 i1 5 test.t7.uq 1 100.00
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Warnings:
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Note 1276 Field or reference 'test.t6.clinic_uq' of SELECT #2 was resolved in SELECT #1
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Note 1003 select `test`.`t6`.`patient_uq` AS `patient_uq`,`test`.`t6`.`clinic_uq` AS `clinic_uq` from `test`.`t7` join `test`.`t6` where `test`.`t6`.`clinic_uq` = `test`.`t7`.`uq`
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@ -4408,7 +4408,7 @@ CREATE INDEX I2 ON t1 (b);
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EXPLAIN SELECT a,b FROM t1 WHERE b IN (SELECT a FROM t1);
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 index I1 I1 2 NULL 2 Using where; Using index; LooseScan
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1 PRIMARY t1 ref I2 I2 13 test.t1.a 2 Using index condition
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1 PRIMARY t1 ref I2 I2 13 test.t1.a 1 Using index condition
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SELECT a,b FROM t1 WHERE b IN (SELECT a FROM t1);
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a b
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CREATE TABLE t2 (a VARCHAR(1), b VARCHAR(10));
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@ -4418,14 +4418,14 @@ CREATE INDEX I2 ON t2 (b);
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EXPLAIN SELECT a,b FROM t2 WHERE b IN (SELECT a FROM t2);
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t2 index I1 I1 4 NULL 2 Using where; Using index; LooseScan
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1 PRIMARY t2 ref I2 I2 13 test.t2.a 2 Using index condition
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1 PRIMARY t2 ref I2 I2 13 test.t2.a 1 Using index condition
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SELECT a,b FROM t2 WHERE b IN (SELECT a FROM t2);
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a b
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EXPLAIN
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SELECT a,b FROM t1 WHERE b IN (SELECT a FROM t1 WHERE LENGTH(a)<500);
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 index I1 I1 2 NULL 2 Using where; Using index; LooseScan
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1 PRIMARY t1 ref I2 I2 13 test.t1.a 2 Using index condition
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1 PRIMARY t1 ref I2 I2 13 test.t1.a 1 Using index condition
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SELECT a,b FROM t1 WHERE b IN (SELECT a FROM t1 WHERE LENGTH(a)<500);
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a b
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DROP TABLE t1,t2;
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@ -6657,7 +6657,7 @@ SET @@optimizer_switch='semijoin=off,materialization=off,in_to_exists=on,subquer
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EXPLAIN SELECT * FROM t1 WHERE a IN (SELECT a AS field1 FROM t1 GROUP BY field1);
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 index NULL a 4 NULL 2 Using where; Using index
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2 DEPENDENT SUBQUERY t1 index_subquery a a 4 func 2 Using index
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2 DEPENDENT SUBQUERY t1 index_subquery a a 4 func 1 Using index
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SELECT * FROM t1 WHERE a IN (SELECT a AS field1 FROM t1 GROUP BY field1);
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a
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2009-01-01
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@ -6860,7 +6860,7 @@ FROM t1 AS alias1, t1 AS alias2, t1 AS alias3
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WHERE alias1.a = alias2.a OR ('Moscow') IN ( SELECT a FROM t1 );
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY alias1 index a a 19 NULL 11 Using where; Using index
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1 PRIMARY alias2 ref a a 19 test.alias1.a 2 Using index
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1 PRIMARY alias2 ref a a 19 test.alias1.a 1 Using index
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1 PRIMARY alias3 index NULL a 19 NULL 11 Using index; Using join buffer (flat, BNL join)
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2 SUBQUERY t1 index_subquery a a 19 const 1 Using index; Using where
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SELECT MAX( alias2.a )
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@ -7011,7 +7011,7 @@ WHERE SLEEP(0.1) OR c < 'p' OR b = ( SELECT MIN(b) FROM t2 );
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 system NULL NULL NULL NULL 1
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1 PRIMARY t2 ALL b NULL NULL NULL 2 Using where
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1 PRIMARY t3 ref d d 5 test.t2.b 2 Using index
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1 PRIMARY t3 ref d d 5 test.t2.b 1 Using index
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3 SUBQUERY NULL NULL NULL NULL NULL NULL NULL Select tables optimized away
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set @tmp_mdev410=@@global.userstat;
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set global userstat=on;
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@ -7043,7 +7043,7 @@ EXPLAIN SELECT * FROM t1 WHERE EXISTS ( SELECT a FROM t1, t2 WHERE b = a GROUP B
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
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2 SUBQUERY t1 range a a 5 NULL 2 Using where; Using index
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2 SUBQUERY t2 ref b b 5 test.t1.a 2 Using index
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2 SUBQUERY t2 ref b b 5 test.t1.a 1 Using index
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DROP TABLE t1,t2;
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#
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# MDEV-435: Expensive subqueries may be evaluated during optimization in merge_key_fields
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@ -7077,7 +7077,7 @@ EXPLAIN SELECT * FROM t1 WHERE EXISTS ( SELECT a FROM t1, t2 WHERE b = a GROUP B
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
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2 SUBQUERY t1 range a a 5 NULL 2 Using where; Using index
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2 SUBQUERY t2 ref b b 5 test.t1.a 2 Using index
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2 SUBQUERY t2 ref b b 5 test.t1.a 1 Using index
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DROP TABLE t1,t2;
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#
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# MDEV-5991: crash in Item_field::used_tables
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