I was troubleshooting an issue where the Oracle Database Result Cache did not get used when the RESULT_CACHE hint was specified inside a subquery. As the main query has a sysdate inside, the RESULT_CACHE Hint can not be specified for the main query, but only for the subquery.

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SELECT SQ.*, sysdate FROM
(SELECT * FROM tableX, ....) SQ;

When using “/*+ RESULT_CACHE */ inside of Subquery, result cache was not enabled.

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SELECT SQ.*, sysdate FROM
(SELECT /*+ RESULT_CACHE */ * FROM tableX, ....) SQ;

I found out that there is a trace event which can be set for diagnostics.

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ALTER SESSION SET EVENTS '43905 trace name context forever, level 1';

This generated a trace file containing a clue to the reason:

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QKSRC: ViewText[ecode=942] = SELECT  /*+ RESULT_CACHE */ ......

So, it seemed that there was an internal ORA-942 somewhere that prevented result caching from being used. The solution was to qualify the objects inside the subquery. Now the result cache was used correctly:

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SELECT SQ.*, sysdate FROM
(SELECT /*+ RESULT_CACHE */ * FROM ownerY.tableX, ....)