简单说两句Like的优化

sys@mescp> select reverse('123') from dual;REVERSE('123')

--------------------------------

321


1 row selected.

sys@mescp> create table test_like as select object_id,object_name from dba_objects;

Table created.

sys@mescp> create index test_like__name on test_like(object_name);

Index created.

sys@mescp> create index test_like__name_reverse on test_like(reverse(object_name));

Index created.
sys@mescp> analyze table test_like compute statistics for table for all indexes;

Table analyzed.

sys@mescp> set autotrace trace exp

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sys@mescp> select * from test_like where object_name like AS%';


Execution Plan

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0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=655 Bytes=15720)

1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST_LIKE' (Cost=2 Card=655Bytes=15720)

2 1 INDEX (RANGE SCAN) OF 'TEST_LIKE__NAME' (NON-UNIQUE) (Cost=2 Card=118)

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sys@mescp> select * from test_like where object_name like '%%';


Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=6 Card=655 Bytes=15720)

1 0 TABLE ACCESS (FULL) OF 'TEST_LIKE' (Cost=6 Card=655 ytes=15720)

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sys@mescp> select * from test_like where object_name like '%S';

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=6 Card=655 Bytes=15720)

1 0 TABLE ACCESS (FULL) OF 'TEST_LIKE' (Cost=6 Card=655 Bytes=15720)

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sys@mescp> select * from test_like where reverse(object_name)like reverse('%AS');

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=655 Bytes=15720)

1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST_LIKE' (Cost=2 Card=655 Bytes=15720)

2 1 INDEX (RANGE SCAN) OF 'TEST_LIKE__NAME_REVERSE' (NON-UNIQUE) (Cost=2 Card=118)

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