Martin Foster <[EMAIL PROTECTED]> writes:
The one not using sub-queries under EXPLAIN ANALYZE proves itself to be less efficient and have a far higher cost then those with the penalty of a sub-query. Since this seems to be counter to what I have been told in the past, I thought I would bring this forward and get some enlightenment.
The ones with the subqueries are not having to form the full join of W and G; they just pick a few rows out of G and look up the matching W rows.
The "subquery penalty" is nonexistent in this case because the subqueries are not dependent on any variables from the outer query, and so they need be evaluated only once, rather than once per outer-query row which is what I suppose you were expecting. This is reflected in the EXPLAIN output: notice they are shown as InitPlans not SubPlans. The outputs of the InitPlans are essentially treated as constants (shown as $0 in the EXPLAIN output) and the outer plan is approximately what it would be if you'd written WHERE g.field = 'constant' instead of WHERE g.field = (select ...)
regards, tom lane
That would explain it overall. Still, it does seem unusual when one puts in additional code, which most literature warns you about and you actually gain a speed boost.
Thanks!
Martin Foster
Creator/Designer Ethereal Realms
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
