Performance tuning a DECODE() statement in a WHERE clause

问题

I re-wrote a query to reduce the time it takes to pick the records. But still I see that there is a small tuning that needs to be done in the decode line as the cost is high. Can someone please let me know if the same query can be re-written without the decode functionality? The purpose of removing the decode function would be to use the index in v_id column.

What I have tried so far.

Tried creating a function index (Knowing that bind variables cannot be used) and it failed.
Have tried using the OR condition but it would not pick the index. So any suggestion would be of a great help.

Query is given below.:

SELECT SUM(NVL(dd.amt,0))
  FROM db,dd
WHERE db.id = dd.dsba_id
  AND dd.nd_id = xxxxxxx
  AND dd.a_id = 'xxxxx-xx'
  AND DECODE (db.v_id , xxxxxxxxx, 'COMPLETE' , db.code ) = 'COMPLETE'
  AND db.datet BETWEEN TRUNC ( SYSDATE , 'YEAR' ) AND SYSDATE;

回答1:

I would suggest writing the code as:

SELECT SUM(dd.amt)
FROM db JOIN
     dd
     ON db.id = dd.dsba_id
WHERE dd.nd_id = xxxxxxx AND
      dd.a_id = 'xxxxx-xx' AND
      (db.v_id = xxxxxxxxx OR db.code = 'COMPLETE') AND
      db.datet >= trunc(sysdate, 'YEAR');

For this query, I would recommend indexes on:

db(nd_id, a_id, id, datet, code)
dd(dsba_id, datet, v_id)

The changes to the above query:

Never use commas in the FROM clause. Always use proper, explicit, standard, readable JOIN syntax. (This does not affect performance, however.)
decode() is rather hard to follow. A simple boolean or is equivalent.
BETWEEN is unnecessary assuming that datet is not in the future.
SUM(NVL()) is not needed, because NULL values are ignored. If you are concerned about NULL result, I would suggest COALESCE(SUM(dd.amt), 0)

来源：https://stackoverflow.com/questions/65580294/performance-tuning-a-decode-statement-in-a-where-clause

标签

sql

Oracle

performance

query-optimization