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Asked: 2026-05-10T18:58:20+00:00

I’ve just discovered that Oracle lets you do the following: SELECT foo.a, (SELECT c

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I’ve just discovered that Oracle lets you do the following:

SELECT foo.a, (SELECT c                 FROM bar                 WHERE foo.a = bar.a)  from foo

As long as only one row in bar matches any row in foo.

The explain plan I get from PL/SQL developer is this:

SELECT STATEMENT, GOAL = ALL_ROWS                 TABLE ACCESS FULL  BAR   TABLE ACCESS FULL  FOO

This doesn’t actually specify how the tables are joined. A colleague asserted that this is more efficient than doing a regular join. Is that true? What is the join strategy on such a select statement, and why doesn’t it show up in the explain plan?

Thanks.

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  1. The plan you have there does not provide much information at all.

    Use SQL*Plus and use dbms_xplan to get a more detailed plan. Look for a script called utlxpls.sql.

    This gives a bit more information:-

    -------------------------------------------------------------------------- | Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     | -------------------------------------------------------------------------- |   0 | SELECT STATEMENT  |      |  1837 | 23881 |     3   (0)| 00:00:01 | |*  1 |  TABLE ACCESS FULL| BAR  |    18 |   468 |     2   (0)| 00:00:01 | |   2 |  TABLE ACCESS FULL| FOO  |  1837 | 23881 |     3   (0)| 00:00:01 | --------------------------------------------------------------------------  Predicate Information (identified by operation id): ---------------------------------------------------     1 - filter('BAR'.'A'=:B1)  Note -----    - dynamic sampling used for this statement  18 rows selected.

    I didn’t create any indexes or foreign keys or collect statistics on the tables, which would change the plan and the join mechanism choosen. Oracle is actually doing a NESTED LOOPS type join here. Step 1, your inline sub-select, is performed for every row returned from FOO.

    This way of performing a SELECT is not quicker. It could be the same or slower. In general try and join everything in the main WHERE clause unless it becomes horribly unreadable.

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