improve performance on WHERE "ABCDEFGHIJK" AS FIELD | '%'

Hi all

I have several tables with a constant number of cases (between 1000 and 1 million) and an indexed field (usually a primary key) I need mark from a PL/SQL function. The database is 11.2.0.2.0 on RedHat.
To make it simple guess this:
CREATE TABLE A (A VARCHAR2(20) CONSTRAINT A_PKEY PRIMARY KEY);
INSERT INTO A VALUES ('ABCD');
INSERT INTO A VALUES ('ABCDEF');
INSERT INTO A VALUES ('ABCDEFG');
INSERT INTO A VALUES ('ABCDEFGH');
INSERT INTO A VALUES ('AACDE');
INSERT INTO A VALUES ('AACDEFG');
INSERT INTO A VALUES ('AACCDDD');
-- between 1000 and 1 million inserts
COMMIT;
ANALYZE TABLE A COMPUTE STATISTICS;
I need the file with the longest field value corresponding to the front of a particular value, for example "ABCDEFGHIJKLMN". The correct result is 'ABCDEFGH '.
A simple solution is this:
SELECT * FROM (
  SELECT * FROM A WHERE 'ABCDEFGHIJKLMN' LIKE A || '%' ORDER BY LENGTH(A) DESC
) WHERE ROWNUM = 1;
The problem is that the DB is not efficiently using the index (EXPLAIN the PLAN said Index Full Table Scan or Full Table Scan).
My real solution is to query from the longest possible values:
DECLARE
   RESULT VARCHAR(20);
BEGIN   
FOR LEN_PREFIX IN REVERSE 4..LENGTH('ABCDEFGHIJKLMN') LOOP
    BEGIN
        SELECT A INTO RESULT FROM A WHERE A=SUBSTR('ABCDEFGHIJKLMN',1,LEN_PREFIX);
        EXIT;
    EXCEPTION        
        WHEN NO_DATA_FOUND THEN
            RESULT := NULL;
    END;
END LOOP;
-- Do something with RESULT (it can be NULL too)
DBMS_OUTPUT.PUT_LINE('RESULT: ' || RESULT);
END;
Is to use the primary key well and the query is compiled (not dynamic, it is located in a Package + PL/SQL function).
What I don't like is that I'm running several queries (end with one exception) before obtaining the final result of "ABCDEFGH".
Since I was 50 million to different values of 'ABCDEFGHIJKLMN', to the query it is increase the number of queries per about 5 times (or more)!

Perhaps another simple solution is to place all the values in a list and then apply the ORDER BY to get the longest value:
SELECT MAX(A) KEEP (DENSE_RANK LAST ORDER BY LENGTH(A)) FROM A
WHERE A IN ('ABCDEFGHIJKLMN', 'ABCDEFGHIJKLM', 'ABCDEFGHIJKL', ... , 'ABCD');
Explain the Plan said SORTING (ITERATOR INLIST (INDEX UNIQUE SCAN))).
Because I want a compiled query I need to replace this list in a nested table or other objects (not sure which is better).
But this solution has the disadvantage that any Oracle internally makes a query for each possible value such as the explain Plan tells me. While with the LOOP FOR that above, I'm from the longest to the first found value.

Here's the code using a nested table:
CREATE TYPE A_TABLE_TYPE IS TABLE OF VARCHAR2(20);
/
DECLARE 
     ALIST A_TABLE_TYPE := A_TABLE_TYPE();
     RESULT VARCHAR(20);
BEGIN
FOR LEN_PREFIX IN 4..LENGTH('ABCDEFGHIJKLMN') LOOP
    ALIST.EXTEND;
    ALIST(LEN_PREFIX-3) := SUBSTR('ABCDEFGHIJKLMN',1,LEN_PREFIX);
END LOOP;
SELECT MAX(A) KEEP (DENSE_RANK LAST ORDER BY LENGTH(A)) INTO RESULT FROM A
WHERE A IN (SELECT * FROM TABLE(ALIST));
-- Do something with RESULT 
DBMS_OUTPUT.PUT_LINE('RESULT: ' || RESULT);
END;
I don't know if a nested query is preferred instead of the DENSE_RANK (I need other fields in the record).
SELECT * FROM (
  SELECT * FROM A WHERE A IN (SELECT * FROM TABLE(ALIST))) ORDER BY LENGTH(A) DESC
) WHERE ROWNUM=1;
Can I add fields, indexes, or whatever is needed to improve performance. Not sure if a partition is useful since they are small tables in any case (although with the number of records).

Any help is welcome,

byeee
Andrea

Published by: 888953 on October 3, 2011 05:27 
with tst
as (select substr('ABCDEFGHIJKLMN',1,4+rownum) val, rownum rn
      from dual
     connect by level<= (length('ABCDEFGHIJKLMN')-4)
    )
  select *
    from (select (select a
                   from a
                  where a=e.val ) res
            from (select val
                    from tst
                   order by rn desc
                 ) e
         )
   where res is not null
      and rownum<=1

Edited by: user6806750 the 03.10.2011 01:29

Tags: Database

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