Improve performance of tuple conversion map generation

Previously convert_tuples_by_name_map naively performed a search of each
outdesc column starting at the first column in indesc and searched each
indesc column until a match was found.  When partitioned tables had many
columns this could result in slow generation of the tuple conversion maps.
For INSERT and UPDATE statements that touched few rows, this could mean a
very large overhead indeed.

We can do a bit better with this loop.  It's quite likely that the columns
in partitioned tables and their partitions are in the same order, so it
makes sense to start searching for each column outer column at the inner
column position 1 after where the previous match was found (per idea from
Alexander Kuzmenkov). This makes the best case search O(N) instead of
O(N^2).  The worst case is still O(N^2), but it seems unlikely that would
happen.

Likewise, in the planner, make_inh_translation_list's search for the
matching column could often end up falling back on an O(N^2) type search.
This commit also improves that by first checking the column that follows
the previous match, instead of the column with the same attnum.  If we
fail to match here we fallback on the syscache's hashtable lookup.

Author: David Rowley
Reviewed-by: Alexander Kuzmenkov
Discussion: https://www.postgresql.org/message-id/CAKJS1f9-wijVgMdRp6_qDMEQDJJ%2BA_n%3DxzZuTmLx5Fz6cwf%2B8A%40mail.gmail.com

src/backend/access/common/tupconvert.c

@@ -295,12 +295,16 @@ convert_tuples_by_name_map(TupleDesc indesc,
 						   const char *msg)
 {
 	AttrNumber *attrMap;
-	int			n;
+	int			outnatts;
+	int			innatts;
 	int			i;
+	int			nextindesc = -1;
 
-	n = outdesc->natts;
-	attrMap = (AttrNumber *) palloc0(n * sizeof(AttrNumber));
-	for (i = 0; i < n; i++)
+	outnatts = outdesc->natts;
+	innatts = indesc->natts;
+
+	attrMap = (AttrNumber *) palloc0(outnatts * sizeof(AttrNumber));
+	for (i = 0; i < outnatts; i++)
 	{
 		Form_pg_attribute outatt = TupleDescAttr(outdesc, i);
 		char	   *attname;
@@ -313,10 +317,27 @@ convert_tuples_by_name_map(TupleDesc indesc,
 		attname = NameStr(outatt->attname);
 		atttypid = outatt->atttypid;
 		atttypmod = outatt->atttypmod;
-		for (j = 0; j < indesc->natts; j++)
+
+		/*
+		 * Now search for an attribute with the same name in the indesc. It
+		 * seems likely that a partitioned table will have the attributes in
+		 * the same order as the partition, so the search below is optimized
+		 * for that case.  It is possible that columns are dropped in one of
+		 * the relations, but not the other, so we use the 'nextindesc'
+		 * counter to track the starting point of the search.  If the inner
+		 * loop encounters dropped columns then it will have to skip over
+		 * them, but it should leave 'nextindesc' at the correct position for
+		 * the next outer loop.
+		 */
+		for (j = 0; j < innatts; j++)
 		{
-			Form_pg_attribute inatt = TupleDescAttr(indesc, j);
+			Form_pg_attribute inatt;
 
+			nextindesc++;
+			if (nextindesc >= innatts)
+				nextindesc = 0;
+
+			inatt = TupleDescAttr(indesc, nextindesc);
 			if (inatt->attisdropped)
 				continue;
 			if (strcmp(attname, NameStr(inatt->attname)) == 0)
@@ -330,7 +351,7 @@ convert_tuples_by_name_map(TupleDesc indesc,
 									   attname,
 									   format_type_be(outdesc->tdtypeid),
 									   format_type_be(indesc->tdtypeid))));
-				attrMap[i] = (AttrNumber) (j + 1);
+				attrMap[i] = inatt->attnum;
 				break;
 			}
 		}
@@ -343,7 +364,6 @@ convert_tuples_by_name_map(TupleDesc indesc,
 							   format_type_be(outdesc->tdtypeid),
 							   format_type_be(indesc->tdtypeid))));
 	}
-
 	return attrMap;
 }
 

src/backend/optimizer/prep/prepunion.c

@@ -51,6 +51,7 @@
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
 #include "utils/selfuncs.h"
+#include "utils/syscache.h"
 
 
 typedef struct
@@ -1895,9 +1896,11 @@ make_inh_translation_list(Relation oldrelation, Relation newrelation,
 	List	   *vars = NIL;
 	TupleDesc	old_tupdesc = RelationGetDescr(oldrelation);
 	TupleDesc	new_tupdesc = RelationGetDescr(newrelation);
+	Oid			new_relid = RelationGetRelid(newrelation);
 	int			oldnatts = old_tupdesc->natts;
 	int			newnatts = new_tupdesc->natts;
 	int			old_attno;
+	int			new_attno = 0;
 
 	for (old_attno = 0; old_attno < oldnatts; old_attno++)
 	{
@@ -1906,7 +1909,6 @@ make_inh_translation_list(Relation oldrelation, Relation newrelation,
 		Oid			atttypid;
 		int32		atttypmod;
 		Oid			attcollation;
-		int			new_attno;
 
 		att = TupleDescAttr(old_tupdesc, old_attno);
 		if (att->attisdropped)
@@ -1939,29 +1941,25 @@ make_inh_translation_list(Relation oldrelation, Relation newrelation,
 		 * Otherwise we have to search for the matching column by name.
 		 * There's no guarantee it'll have the same column position, because
 		 * of cases like ALTER TABLE ADD COLUMN and multiple inheritance.
-		 * However, in simple cases it will be the same column number, so try
-		 * that before we go groveling through all the columns.
-		 *
-		 * Note: the test for (att = ...) != NULL cannot fail, it's just a
-		 * notational device to include the assignment into the if-clause.
-		 */
-		if (old_attno < newnatts &&
-			(att = TupleDescAttr(new_tupdesc, old_attno)) != NULL &&
-			!att->attisdropped &&
-			strcmp(attname, NameStr(att->attname)) == 0)
-			new_attno = old_attno;
-		else
-		{
-			for (new_attno = 0; new_attno < newnatts; new_attno++)
+		 * However, in simple cases, the relative order of columns is mostly
+		 * the same in both relations, so try the column of newrelation that
+		 * follows immediately after the one that we just found, and if that
+		 * fails, let syscache handle it.
+		 */
+		if (new_attno >= newnatts ||
+			(att = TupleDescAttr(new_tupdesc, new_attno))->attisdropped ||
+			strcmp(attname, NameStr(att->attname)) != 0)
 		{
-				att = TupleDescAttr(new_tupdesc, new_attno);
-				if (!att->attisdropped &&
-					strcmp(attname, NameStr(att->attname)) == 0)
-					break;
-			}
-			if (new_attno >= newnatts)
+			HeapTuple	newtup;
+
+			newtup = SearchSysCacheAttName(new_relid, attname);
+			if (!newtup)
 				elog(ERROR, "could not find inherited attribute \"%s\" of relation \"%s\"",
 					 attname, RelationGetRelationName(newrelation));
+			new_attno = ((Form_pg_attribute) GETSTRUCT(newtup))->attnum - 1;
+			ReleaseSysCache(newtup);
+
+			att = TupleDescAttr(new_tupdesc, new_attno);
 		}
 
 		/* Found it, check type and collation match */
@@ -1978,6 +1976,7 @@ make_inh_translation_list(Relation oldrelation, Relation newrelation,
 									 atttypmod,
 									 attcollation,
 									 0));
+		new_attno++;
 	}
 
 	*translated_vars = vars;