教你快速掌握Oracle数据库中的like优化-性能调优
1。尽量不要使用 like '%..%'
   
    2。对于 like '..%..' (不以 % 开头)，Oracle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:

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

    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

    -- '常量开头的like , 会利用index ，没问题…… '
    sys@mescp> 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'
    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)

    -- '以常量结束，直接写的时候是不能应用index的'
    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)

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