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- First code. Elimination works for simple cases, passes the testsuite. - Known issues: = No elimination is done for aggregate functions. = EXPLAIN EXTENDED shows eliminated tables (I think it better not) = No benchmark yet = The code needs some polishing. mysql-test/r/table_elim.result: MWL#17: Table elimination - Testcases mysql-test/t/table_elim.test: MWL#17: Table elimination - Testcases sql/sql_select.cc: MWL#17: Table elimination sql/sql_select.h: MWL#17: Table elimination - Added JOIN_TAB::eliminated (is JOIN_TAB the best place to store this flag?) sql/table.h: MWL#17: Table elimination - ADded NESTED_JOIN::n_tables. We need to have the number of real tables remaining in an outer join nest.
56 lines
2.5 KiB
Text
56 lines
2.5 KiB
Text
drop table if exists t0, t1, t2, t3;
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create table t1 (a int);
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insert into t1 values (0),(1),(2),(3);
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create table t0 as select * from t1;
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create table t2 (a int primary key, b int)
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as select a, a as b from t1 where a in (1,2);
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create table t3 (a int primary key, b int)
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as select a, a as b from t1 where a in (1,3);
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# This will be eliminated:
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explain select t1.a from t1 left join t2 on t2.a=t1.a;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 ALL NULL NULL NULL NULL 4
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select t1.a from t1 left join t2 on t2.a=t1.a;
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a
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0
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1
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2
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3
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# This will not be eliminated as t2.b is in in select list:
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explain select * from t1 left join t2 on t2.a=t1.a;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 ALL NULL NULL NULL NULL 4
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1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t1.a 1
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# This will not be eliminated as t2.b is in in order list:
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explain select t1.a from t1 left join t2 on t2.a=t1.a order by t2.b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 ALL NULL NULL NULL NULL 4 Using temporary; Using filesort
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1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t1.a 1
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# This will not be eliminated as t2.b is in group list:
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explain select t1.a from t1 left join t2 on t2.a=t1.a group by t2.b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 ALL NULL NULL NULL NULL 4 Using temporary; Using filesort
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1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t1.a 1
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# This will not be eliminated as t2.b is in the WHERE
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explain select t1.a from t1 left join t2 on t2.a=t1.a where t2.b < 3 or t2.b is null;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 ALL NULL NULL NULL NULL 4
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1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t1.a 1 Using where
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# Elimination of multiple tables:
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explain select t1.a from t1 left join (t2 join t3) on t2.a=t1.a and t3.a=t1.a;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 ALL NULL NULL NULL NULL 4
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# Elimination of multiple tables (2):
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explain select t1.a from t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and t3.a=t1.a;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 ALL NULL NULL NULL NULL 4
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# Elimination when done within an outer join nest:
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explain
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select t0.*
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from
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t0 left join (t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
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t3.a=t1.a) on t0.a=t1.a;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t0 ALL NULL NULL NULL NULL 4
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1 SIMPLE t1 ALL NULL NULL NULL NULL 4
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drop table t0, t1, t2, t3;
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