Cache the results of format_type() queries in pg_dump.

tglsfdc · tglsfdc · commit 6c450a861f1a · 2021-08-31T13:53:49.000-04:00
There's long been a "TODO: there might be some value in caching the results" annotation on pg_dump's getFormattedTypeName function; but we hadn't gotten around to checking what it was costing us to repetitively look up type names. It turns out that when dumping the current regression database, about 10% of the total number of queries issued are duplicative format_type() queries. However, Hubert Depesz Lubaczewski reported a not-unusual case where these account for over half of the queries issued by pg_dump. Individually these queries aren't expensive, but when network lag is a factor, they add up to a problem. We can very easily add some caching to getFormattedTypeName to solve it. Since this is such a simple fix and can have a visible performance benefit, back-patch to all supported branches. Discussion: https://postgr.es/m/20210826084430.GA26282@depesz.com
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
@@ -5232,6 +5232,7 @@ getTypes(Archive *fout, int *numTypes)
 		tyinfo[i].dobj.name = pg_strdup(PQgetvalue(res, i, i_typname));
 		tyinfo[i].dobj.namespace =
 			findNamespace(atooid(PQgetvalue(res, i, i_typnamespace)));
+		tyinfo[i].ftypname = NULL;	/* may get filled later */
 		tyinfo[i].rolname = pg_strdup(PQgetvalue(res, i, i_rolname));
 		tyinfo[i].typacl = pg_strdup(PQgetvalue(res, i, i_typacl));
 		tyinfo[i].rtypacl = pg_strdup(PQgetvalue(res, i, i_rtypacl));
@@ -18892,12 +18893,11 @@ findDumpableDependencies(ArchiveHandle *AH, const DumpableObject *dobj,
  *
  * This does not guarantee to schema-qualify the output, so it should not
  * be used to create the target object name for CREATE or ALTER commands.
- *
- * TODO: there might be some value in caching the results.
  */
 static char *
 getFormattedTypeName(Archive *fout, Oid oid, OidOptions opts)
 {
+	TypeInfo   *typeInfo;
 	char	   *result;
 	PQExpBuffer query;
 	PGresult   *res;
@@ -18910,6 +18910,11 @@ getFormattedTypeName(Archive *fout, Oid oid, OidOptions opts)
 			return pg_strdup("NONE");
 	}
 
+	/* see if we have the result cached in the type's TypeInfo record */
+	typeInfo = findTypeByOid(oid);
+	if (typeInfo && typeInfo->ftypname)
+		return pg_strdup(typeInfo->ftypname);
+
 	query = createPQExpBuffer();
 	appendPQExpBuffer(query, "SELECT pg_catalog.format_type('%u'::pg_catalog.oid, NULL)",
 					  oid);
@@ -18922,6 +18927,10 @@ getFormattedTypeName(Archive *fout, Oid oid, OidOptions opts)
 	PQclear(res);
 	destroyPQExpBuffer(query);
 
+	/* cache a copy for later requests */
+	if (typeInfo)
+		typeInfo->ftypname = pg_strdup(result);
+
 	return result;
 }
 
diff --git a/src/bin/pg_dump/pg_dump.h b/src/bin/pg_dump/pg_dump.h
@@ -166,9 +166,11 @@ typedef struct _typeInfo
 	DumpableObject dobj;
 
 	/*
-	 * Note: dobj.name is the pg_type.typname entry.  format_type() might
-	 * produce something different than typname
+	 * Note: dobj.name is the raw pg_type.typname entry.  ftypname is the
+	 * result of format_type(), which will be quoted if needed, and might be
+	 * schema-qualified too.
 	 */
+	char	   *ftypname;
 	char	   *rolname;		/* name of owner, or empty string */
 	char	   *typacl;
 	char	   *rtypacl;
