掌握Oracle資料庫中的like優化-效能調優
    1.儘量不要使用 like '%..%'
    2.對於 like '..%..' (不以 % 開頭)，Oracle可以應用 colunm上的index
    3.對於 like '%...' 的 (不以 % 結尾)，可以利用reverse + function index 的形式，變化成 like '..%'
    程式碼:
    -- '建測試表和Index，注意，重點在於帶reverse的function index。同時，一定要使用CBO才行……
    [email protected]> select reverse('123') from dual;REVERSE('123')
    --------------------------------
    321
    1 row selected.
   

[email protected]> create table test_like as select object_id,object_name from dba_objects;
    Table created.
    [email protected]> create index test_like__name on test_like(object_name);
    Index created.
    [email protected]> create index test_like__name_reverse on test_like(reverse(object_name));
    Index created.
   

[email protected]> analyze table test_like compute statistics for table for all indexes;
    Table analyzed.
    [email protected]> set autotrace trace exp

-- '常量開頭的like , 會利用index ，沒問題…… '
    [email protected]> 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)
    --'開頭和結尾都是 % ,對不起，很難優化'
   

[email protected]> 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)
    -- '以常量結束，直接寫的時候是不能應用index的'
    [email protected]> 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)
    --'以常量結束的，加個reverse 函式，又可以用上index了'
    [email protected]> 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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