Tag Archives: like

How to tune WHERE NAME LIKE ‘%ABC%’

More than once customers wanted me to tune queries where the LIKE clause made the query very slow…

The easy answer is : you cannot. If you want to search for some characters in the middle of the string, Oracle will not use an index.

Is it a correct answer? Maybe not.

Let’s imagine my application allow you to search for a string in the middle of the name, but to avoid scanning too large amount of data, the application enforces the string to be at least 3 characters long. For example ABC.

In this case, instead of doing a full table scan of the table to retrieve only a few rows, we can use an hint to tell Oracle to use an index on the name:

CREATE TABLE lsc_t AS
SELECT
  ROWNUM ID,
  SUBSTR(DBMS_RANDOM.STRING ('n', DBMS_RANDOM.VALUE (2, 8))
    || ' '
    || DBMS_RANDOM.STRING ('n', DBMS_RANDOM.VALUE (2, 8))
    ,1,17) NAME,
  TRUNC(SYSDATE-ABS(10000*DBMS_RANDOM.NORMAL)) birthdate,
  LPAD('X',4000,'X') address
FROM DUAL
CONNECT BY LEVEL <= 1e5;

ALTER TABLE lsc_t ADD PRIMARY KEY(ID);

CREATE INDEX lsc_i ON lsc_t(NAME);

EXEC dbms_stats.gather_table_stats(user,'LSC_T',cascade=>true)

let’s measure the time for a full table scan

SQL> set timi on
SQL> SELECT ID, NAME, birthdate
  FROM lsc_t WHERE NAME LIKE '%ABC%';

       ID NAME              BIRTHDATE
--------- ----------------- ---------
    60249 ABCBIFAB KRKBCRN  11-MAR-90
    16714 AF YABCG          09-OCT-95
    55571 BABCIQ GESGLW     27-MAR-50
    77561 BP GABC           24-APR-90
    80027 DALSABC TZLOAWDV  05-NOV-01
    49817 EABCTFIY XWB      10-FEB-88
    23283 EMMOGGBF DABCB    20-DEC-87
    39530 FMABCKB AB        18-SEP-87
    68605 FTPGOHE ABCC      28-SEP-28
    74615 KIFDWABC CSSUQ    08-AUG-82
    31772 KNOABCT BO        08-SEP-77
    68730 KRYIEN LMABC      10-APR-07
    43317 LUFJKZJT AUABCZR  19-DEC-88
    76851 MZABC TEIFG       14-SEP-92
    54589 NXE YABCDX        03-MAY-88
     6940 OIWABCZ DLFFXY    29-MAR-88
    59070 ONIB ADGABCI      29-JUL-07
    27264 PGHOABC ZY        05-OCT-90
    38157 QABC OPZHE        13-JUN-87
    17511 QPDKD CIABCJ      08-AUG-69
    25507 RX OWULOABC       24-FEB-92
    62159 SEABC DAILK       25-JUN-02
     3845 SK CCABCG         22-JAN-80
    50059 SPABC BVHRHW      18-MAR-86
    54700 UABCPC WUHAJS     28-OCT-71
    70207 UKY OIDUABC       23-APR-88
    39484 WABC TJLYHVJZ     14-MAR-78
    14561 WDRWABC XZKDH     29-MAR-86
    61501 YBYU RYABCGI      28-JUN-78
    30578 YEWENGX ABCHARA   12-SEP-67
    35397 YHBEABC HFKO      25-AUG-85
    26450 YOABCVG HJT       23-DEC-98
    87224 ZKNLNY YAABC      13-NOV-61

33 rows selected.

Elapsed: 00:00:02.56

about 3 seconds for retrieving 33 rows out of 100000

let’s try with an index

SQL> SELECT /*+INDEX(LSC_T,LSC_I)*/  ID, NAME, birthdate
  FROM lsc_t WHERE NAME LIKE '%ABC%';

       ID NAME              BIRTHDATE
--------- ----------------- ---------
    60249 ABCBIFAB KRKBCRN  11-MAR-90
    16714 AF YABCG          09-OCT-95
    55571 BABCIQ GESGLW     27-MAR-50
    77561 BP GABC           24-APR-90
    80027 DALSABC TZLOAWDV  05-NOV-01
    49817 EABCTFIY XWB      10-FEB-88
    23283 EMMOGGBF DABCB    20-DEC-87
    39530 FMABCKB AB        18-SEP-87
    68605 FTPGOHE ABCC      28-SEP-28
    74615 KIFDWABC CSSUQ    08-AUG-82
    31772 KNOABCT BO        08-SEP-77
    68730 KRYIEN LMABC      10-APR-07
    43317 LUFJKZJT AUABCZR  19-DEC-88
    76851 MZABC TEIFG       14-SEP-92
    54589 NXE YABCDX        03-MAY-88
     6940 OIWABCZ DLFFXY    29-MAR-88
    59070 ONIB ADGABCI      29-JUL-07
    27264 PGHOABC ZY        05-OCT-90
    38157 QABC OPZHE        13-JUN-87
    17511 QPDKD CIABCJ      08-AUG-69
    25507 RX OWULOABC       24-FEB-92
    62159 SEABC DAILK       25-JUN-02
     3845 SK CCABCG         22-JAN-80
    50059 SPABC BVHRHW      18-MAR-86
    54700 UABCPC WUHAJS     28-OCT-71
    70207 UKY OIDUABC       23-APR-88
    39484 WABC TJLYHVJZ     14-MAR-78
    14561 WDRWABC XZKDH     29-MAR-86
    61501 YBYU RYABCGI      28-JUN-78
    30578 YEWENGX ABCHARA   12-SEP-67
    35397 YHBEABC HFKO      25-AUG-85
    26450 YOABCVG HJT       23-DEC-98
    87224 ZKNLNY YAABC      13-NOV-61

33 rows selected.

Elapsed: 00:00:00.06

Much better :)

reposted due to % in url