How to optimize an update SQL that runs on a Oracle table with 700M rows

Question

UPDATE [TABLE] SET [FIELD]=0 WHERE [FIELD] IS NULL

[TABLE] is an Oracle database table with more than 700 million rows. I cancelled the SQL execution a

Answer 1

Are other users are updating the same rows in the table at the same time ?

If so, you could be hitting lots of concurrency issues (waiting for locks) and it may be worth breaking it into smaller transactions.

DECLARE
  v_cnt number := 1;
BEGIN
 WHILE v_cnt > 0 LOOP
   UPDATE [TABLE] SET [FIELD]=0 WHERE [FIELD] IS NULL AND ROWNUM < 50000;
   v_cnt := SQL%ROWCOUNT;
   COMMIT;
 END LOOP;
END;
/

The smaller the ROWNUM limit the less concurrency/locking issues you'll hit, but the more time you'll spend in table scanning.

Answer 2

Some suggestions:

1. Drop any indexes that contain FIELD before running your UPDATE statement, and then re-add them later.

2. Write a PL/SQL procedure to do this that commits after every 1000 or 10000 rows.

Hope this helps.

Answer 3

First of all is it a one-time query or is it a recurrent query ? If you only have to do it once you may want to look into running the query in parallel mode. You will have to scan all rows anyway, you could either divide the workload yourself with ranges of ROWID (do-it-yourself parallelism) or use Oracle built-in features.

Assuming you want to run it frequently and want to optimize this query, the number of rows with the field column as NULL will eventually be small compared to the total number of rows. In that case an index could speed things up. Oracle doesn't index rows that have all indexed columns as NULL so an index on field won't get used by your query (since you want to find all rows where field is NULL).

Either:

• create an index on (FIELD, 0), the 0 will act as a non-NULL pseudocolumn and all rows will be indexed on the table.

• create a function-based index on (CASE WHEN field IS NULL THEN 1 END), this will only index the rows that are NULLs (the index would therefore be very compact). In that case you would have to rewrite your query:

  UPDATE [TABLE] SET [FIELD]=0 WHERE (CASE WHEN field IS NULL THEN 1 END)=1

Edit:

Since this is a one-time scenario, you may want to use the PARALLEL hint:

SQL> EXPLAIN PLAN FOR
  2  UPDATE /*+ PARALLEL(test_table 4)*/ test_table
  3     SET field=0
  4   WHERE field IS NULL;

Explained

SQL> select * from table( dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 4026746538
--------------------------------------------------------------------------------
| Id  | Operation             | Name       | Rows  | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT      |            | 22793 |   289K|    12   (9)| 00:00:
|   1 |  UPDATE               | TEST_TABLE |       |       |            |
|   2 |   PX COORDINATOR      |            |       |       |            |
|   3 |    PX SEND QC (RANDOM)| :TQ10000   | 22793 |   289K|    12   (9)| 00:00:
|   4 |     PX BLOCK ITERATOR |            | 22793 |   289K|    12   (9)| 00:00:
|*  5 |      TABLE ACCESS FULL| TEST_TABLE | 22793 |   289K|    12   (9)| 00:00:
--------------------------------------------------------------------------------

Answer 4

You could acheive the same result without updating by using an ALTER table to set the columns "DEFAULT" value to 0.

Answer 5

Vincent already answered your question perfectly, but I'm curious about the "why" behind this action. Why are you updating all NULL's to 0?

Regards, Rob.