Fix O(N^2) performance issue in pg_publication_tables view.

tglsfdc · tglsfdc · commit 166f69f769c8 · 2019-05-22T11:47:02.000-04:00
The original coding of this view relied on a correlated IN sub-query. Our planner is not very bright about correlated sub-queries, and even if it were, there's no way for it to know that the output of pg_get_publication_tables() is duplicate-free, making the de-duplicating semantics of IN unnecessary. Hence, rewrite as a LATERAL sub-query. This provides circa 100X speedup for me with a few hundred published tables (the whole regression database), and things would degrade as roughly O(published_relations * all_relations) beyond that. Because the rules.out expected output changes, force a catversion bump. Ordinarily we might not want to do that post-beta1; but we already know we'll be doing a catversion bump before beta2 to fix pg_statistic_ext issues, so it's pretty much free to fix it now instead of waiting for v13. Per report and fix suggestion from PegoraroF10. Discussion: https://postgr.es/m/1551385426763-0.post@n3.nabble.com
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
@@ -258,9 +258,10 @@ CREATE VIEW pg_publication_tables AS
         P.pubname AS pubname,
         N.nspname AS schemaname,
         C.relname AS tablename
-    FROM pg_publication P, pg_class C
-         JOIN pg_namespace N ON (N.oid = C.relnamespace)
-    WHERE C.oid IN (SELECT relid FROM pg_get_publication_tables(P.pubname));
+    FROM pg_publication P,
+         LATERAL pg_get_publication_tables(P.pubname) GPT,
+         pg_class C JOIN pg_namespace N ON (N.oid = C.relnamespace)
+    WHERE C.oid = GPT.relid;
 
 CREATE VIEW pg_locks AS
     SELECT * FROM pg_lock_status() AS L;
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201905141
+#define CATALOG_VERSION_NO	201905221
 
 #endif
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
@@ -1441,10 +1441,10 @@ pg_publication_tables| SELECT p.pubname,
     n.nspname AS schemaname,
     c.relname AS tablename
    FROM pg_publication p,
+    LATERAL pg_get_publication_tables((p.pubname)::text) gpt(relid),
     (pg_class c
      JOIN pg_namespace n ON ((n.oid = c.relnamespace)))
-  WHERE (c.oid IN ( SELECT pg_get_publication_tables.relid
-           FROM pg_get_publication_tables((p.pubname)::text) pg_get_publication_tables(relid)));
+  WHERE (c.oid = gpt.relid);
 pg_replication_origin_status| SELECT pg_show_replication_origin_status.local_id,
     pg_show_replication_origin_status.external_id,
     pg_show_replication_origin_status.remote_lsn,
