On Thu, 30 Sep 2004, Merlin Moncure wrote: > OK, I have a situation that might be a performance problem, a bug, or an > unavoidable consequence of using prepared statements. The short version > is that I am getting function executions for rows not returned in a > result set when they are in a prepared statement. > > In other words, I have a query: > select f(t.c) from t where [boolean expr on t] limit 1;
An actual boolean expr on t? Or on a column in t? > because of the limit phrase, obviously, at most one record is returned > and f executes at most once regardless of the plan used (in practice, > sometimes index, sometimes seq_scan. > > Now, if the same query is executed as a prepared statement, > prepare ps(...) as select f(t.c) from t where [expr] limit 1; > execute ps; > > now, if ps ends up using a index scan on t, everything is ok. However, > if ps does a seqscan, f executes for every row on t examined until the > [expr] criteria is met. Is this a bug? If necessary I should be able > to set up a reproducible example. The easy workaround is to not use > prepared statements in these situations, but I need to be able to > guarantee that f only executes once (even if that means exploring > subqueries). I think a reproducible example would be good. Simple attempts to duplicate this on 8.0b2 have failed for me, unless I'm using order by. ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster
