Fix performance hazard in REFRESH MATERIALIZED VIEW CONCURRENTLY.

tglsfdc · tglsfdc · commit 6fbd5cce22eb · 2018-03-19T17:23:21.000-04:00
Jeff Janes discovered that commit 7ca25b7 made one of the queries run by REFRESH MATERIALIZED VIEW CONCURRENTLY perform badly. The root cause is bad cardinality estimation for correlated quals, but a principled solution to that problem is some way off, especially since the planner lacks any statistics about whole-row variables. Moreover, in non-error cases this query produces no rows, meaning it must be run to completion; but use of LIMIT 1 encourages the planner to pick a fast-start, slow-completion plan, exactly not what we want. Remove the LIMIT clause, and instead rely on the count parameter we pass to SPI_execute() to prevent excess work if the query does return some rows. While we've heard no field reports of planner misbehavior with this query, it could be that people are having performance issues that haven't reached the level of pain needed to cause a bug report. In any case, that LIMIT clause can't possibly do anything helpful with any existing version of the planner, and it demonstrably can cause bad choices in some cases, so back-patch to 9.4 where the code was introduced. Thomas Munro Discussion: https://postgr.es/m/CAMkU=1z-JoGymHneGHar1cru4F1XDfHqJDzxP_CtK5cL3DOfmg@mail.gmail.com
diff --git a/src/backend/commands/matview.c b/src/backend/commands/matview.c
@@ -657,10 +657,10 @@ refresh_by_match_merge(Oid matviewOid, Oid tempOid, Oid relowner,
 	appendStringInfo(&querybuf,
 					 "SELECT newdata FROM %s newdata "
 					 "WHERE newdata IS NOT NULL AND EXISTS "
-					 "(SELECT * FROM %s newdata2 WHERE newdata2 IS NOT NULL "
+					 "(SELECT 1 FROM %s newdata2 WHERE newdata2 IS NOT NULL "
 					 "AND newdata2 OPERATOR(pg_catalog.*=) newdata "
 					 "AND newdata2.ctid OPERATOR(pg_catalog.<>) "
-					 "newdata.ctid) LIMIT 1",
+					 "newdata.ctid)",
 					 tempname, tempname);
 	if (SPI_execute(querybuf.data, false, 1) != SPI_OK_SELECT)
 		elog(ERROR, "SPI_exec failed: %s", querybuf.data);
