Bruce Momjian wrote: > However, for the wire protocol prepare/execute, how do you do EXPLAIN? > The only way I can see doing it is to put the EXPLAIN in the prepare > query, but I wasn't sure that works. So, I just wrote and tested the > attached C program and it properly output the explain information, e.g. > > res = PQprepare(conn, "prep1", "EXPLAIN SELECT * FROM pg_language", 0, > NULL); > ------- > generated: > > QUERY PLAN > > Seq Scan on pg_language (cost=0.00..1.04 rows=4 width=114) > > so that works --- good.
Hm, yes. Were you just curious or is it relevant for the documentation update? >>> Looking at how the code behaves, it seems custom plans that are _more_ >>> expensive (plus planning cost) than the generic plan switch to the >>> generic plan after five executions, as now documented. Custom plans >>> that are significantly _cheaper_ than the generic plan _never_ use the >>> generic plan. >> >> Yes, that's what the suggested documentation improvement says as well, >> right? > > Yes. What is odd is that it isn't the plan of the actual supplied > parameters that is cheaper, just the generic plan that assumes each > distinct value in the query is equally likely to be used. So, when we > say the generic plan is cheaper, it is just comparing the custom plan > with the supplied parameters vs. the generic plan --- it is not saying > that running the supplied constants with the generic plan will execute > faster, because in fact we might be using a sub-optimial generic plan. Right, that's why it is important to document that it is estimates that are compared, not actual costs. This has caused confussion in the past, see https://www.postgresql.org/message-id/flat/561E749D.4090301%40socialserve.com#561e749d.4090...@socialserve.com > Right. Updated patch attached. I am happy with the patch as it is. Yours, Laurenz Albe -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
