Re: IN joining

Dennis Haney <davh(at)diku(dot)dk> writes:
> Consider this example:
> SELECT * FROM a,b WHERE a.id = b.id AND (a.id) IN (SELECT c.id FROM c)
> the possible execution trees are {{a,b}, {c}}, {{a,c},{b}} and the code
> seems to also permit {{b,c},{a}}.

No, it does not --- as you say, that would give wrong answers. That
case is eliminated by the tests following this comment:

* JOIN_IN technique will work if outerrel includes LHS and
* innerrel is exactly RHS; conversely JOIN_REVERSE_IN handles
* RHS/LHS.
*
* JOIN_UNIQUE_OUTER will work if outerrel is exactly RHS;
* conversely JOIN_UNIQUE_INNER will work if innerrel is
* exactly RHS.

Joining {b,c} to {a} does not meet any of those four allowed cases.

regards, tom lane