教你快速掌握Oracle数据库中的like优化

    1。尽量不要使用 like '%..%'
   
    2。对于 like '..%..' (不以 % 开头)，可以应用 colunm上的index
   
    3。对于 like '%...' 的 (不以 % 结尾)，可以利用reverse + function index 的形式，变化成 like '..%'

    代码:

    -- '建表和Index，注意，重点在于带reverse的function index。同时，一定要使用CBO才行……

    > select reverse('123') from dual;REVERSE('123')

    --------------------------------

    321

    1 row selected.

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

    Table created.

    > create index test_like__name on test_like(object_name);

    Index created.

    > create index test_like__name_reverse on test_like(reverse(object_name));

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

    Table analyzed.

    > set autotrace trace exp

    -- '常量开头的like , 会利用index ，没问题…… '

    > select * from test_like where object_name like '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=655Bytes=15720)

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

    --'开头和结尾都是 % ,对不起，很难优化'

    > 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)

    -- '以常量结束，直接写的时候是不能应用index的'

    > 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)

    --'以常量结束的，加个reverse 函数，又可以用上index了'

    > 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)

 

【责编:Ken】

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