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Editorial Team
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Asked: 2026-06-17T14:45:54+00:00

I have a query (below). The explain plan shows high CPU utilization which has

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I have a query (below). The explain plan shows high CPU utilization which has also caused downtime in our lab. So it possible to tube this query further? What should i do to tune it?

FYI, mtr_main_a, mtr_main_b, mtr_hist contain huge no of records may be 10 millions or more.

SELECT to_char(MAX(mdt), 'MM-DD-RRRR HH24:MI:SS')
FROM   (
           SELECT MAX(mod_date - 2 / 86400) mdt
           FROM   mtr_main_a
           UNION
           SELECT MAX(mod_date - 2 / 86400) mdt
           FROM   mtr_main_b
           UNION
           SELECT MAX(mod_date - 2 / 86400) mdt
           FROM   mtr_hist@batch_hist
       )
/

Explain plan is as below

Execution Plan
----------------------------------------------------------
Plan hash value: 1573811822

-------------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name       | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     | Inst   |IN-OUT|
-------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |            |     1 |     9 |       | 79803   (1)| 00:18:38 |        |      |
|   1 |  SORT AGGREGATE        |            |     1 |     9 |       |            |          |        |      |
|   2 |   VIEW                 |            |     2 |    18 |       | 79803   (1)| 00:18:38 |        |      |
|   3 |    SORT UNIQUE         |            |     2 |    17 |    77M| 79803   (2)| 00:18:38 |        |      |
|   4 |     UNION-ALL          |            |       |       |       |            |          |        |      |
|   5 |      SORT AGGREGATE    |            |     1 |     8 |       | 79459   (1)| 00:18:33 |        |      |
|   6 |       TABLE ACCESS FULL| MTR_MAIN_A |  5058K|    38M|       | 67735   (1)| 00:15:49 |        |      |
|   7 |      SORT AGGREGATE    |            |     1 |     9 |       |   344   (1)| 00:00:05 |        |      |
|   8 |       TABLE ACCESS FULL| MTR_MAIN_B |     1 |     9 |       |   343   (1)| 00:00:05 |        |      |
|   9 |      REMOTE            |            |       |       |       |            |          |  HISTB | R->S |
-------------------------------------------------------------------------------------------------------------

Remote SQL Information (identified by operation id):
----------------------------------------------------

   9 - EXPLAIN PLAN SET STATEMENT_ID='PLUS10294704' INTO PLAN_TABLE@! FOR SELECT
       MAX("A1"."MOD_DATE"-.00002314814814814814814814814814814814814815) FROM "MTR_HIST" "A1" (accessing
       'HISTB' )

Thanks and Regards,
Chandra

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1 Answer

  1. Editorial Team

    You should be able to greatly improve the performance by putting an index on the mod_date columns and changing your query in a way that does the subtraction at the end, after you determined the maximum date:

    SELECT to_char(MAX(mdt) - 2 / 86400, 'MM-DD-RRRR HH24:MI:SS')
FROM   (
           SELECT MAX(mod_date) mdt
           FROM   mtr_main_a
           UNION
           SELECT MAX(mod_date) mdt
           FROM   mtr_main_b
           UNION
           SELECT MAX(mod_date) mdt
           FROM   mtr_hist@batch_hist
       )

    This should get rid of the full table scans.

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