Cause planner to make use of average-column-width statistic that is now

collected by ANALYZE.  Also, add some modest amount of intelligence to
guesses that are used for varlena columns in the absence of any ANALYZE
statistics.  The 'width' reported by EXPLAIN is finally something less
than totally bogus for varlena columns ... and, in consequence, hashjoin
estimating should be a little better ...

1 files changed, 43 insertions, 32 deletions

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index bdfbbb18186..7dfe834b779 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -41,7 +41,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.71 2001/05/07 00:43:20 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.72 2001/05/09 00:35:09 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -61,12 +61,6 @@
 #include "utils/syscache.h"
 
 
-/*
- * The length of a variable-length field in bytes (stupid estimate...)
- */
-#define _DEFAULT_ATTRIBUTE_WIDTH_ 12
-
-
 #define LOG2(x)  (log(x) / 0.693147180559945)
 #define LOG6(x)  (log(x) / 1.79175946922805)
 
@@ -90,7 +84,6 @@ bool		enable_hashjoin = true;
 
 static bool cost_qual_eval_walker(Node *node, Cost *total);
 static void set_rel_width(Query *root, RelOptInfo *rel);
-static int	compute_attribute_width(TargetEntry *tlistentry);
 static double relation_byte_size(double tuples, int width);
 static double page_size(double tuples, int width);
 
@@ -1082,36 +1075,54 @@ set_joinrel_size_estimates(Query *root, RelOptInfo *rel,
 /*
  * set_rel_width
  *		Set the estimated output width of the relation.
+ *
+ * NB: this works best on base relations because it prefers to look at
+ * real Vars.  It will fail to make use of pg_statistic info when applied
+ * to a subquery relation, even if the subquery outputs are simple vars
+ * that we could have gotten info for.  Is it worth trying to be smarter
+ * about subqueries?
  */
 static void
 set_rel_width(Query *root, RelOptInfo *rel)
 {
-	int			tuple_width = 0;
-	List	   *tle;
+	int32		tuple_width = 0;
+	List	   *tllist;
 
-	foreach(tle, rel->targetlist)
-		tuple_width += compute_attribute_width((TargetEntry *) lfirst(tle));
-	Assert(tuple_width >= 0);
-	rel->width = tuple_width;
-}
+	foreach(tllist, rel->targetlist)
+	{
+		TargetEntry *tle = (TargetEntry *) lfirst(tllist);
+		int32	item_width;
 
-/*
- * compute_attribute_width
- *	  Given a target list entry, find the size in bytes of the attribute.
- *
- *	  If a field is variable-length, we make a default assumption.	Would be
- *	  better if VACUUM recorded some stats about the average field width...
- *	  also, we have access to the atttypmod, but fail to use it...
- */
-static int
-compute_attribute_width(TargetEntry *tlistentry)
-{
-	int			width = get_typlen(tlistentry->resdom->restype);
+		/*
+		 * If it's a Var, try to get statistical info from pg_statistic.
+		 */
+		if (tle->expr && IsA(tle->expr, Var))
+		{
+			Var	   *var = (Var *) tle->expr;
+			Oid		relid;
 
-	if (width < 0)
-		return _DEFAULT_ATTRIBUTE_WIDTH_;
-	else
-		return width;
+			relid = getrelid(var->varno, root->rtable);
+			if (relid != InvalidOid)
+			{
+				item_width = get_attavgwidth(relid, var->varattno);
+				if (item_width > 0)
+				{
+					tuple_width += item_width;
+					continue;
+				}
+			}
+		}
+		/*
+		 * Not a Var, or can't find statistics for it.  Estimate using
+		 * just the type info.
+		 */
+		item_width = get_typavgwidth(tle->resdom->restype,
+									 tle->resdom->restypmod);
+		Assert(item_width > 0);
+		tuple_width += item_width;
+	}
+	Assert(tuple_width >= 0);
+	rel->width = tuple_width;
 }
 
 /*
@@ -1122,7 +1133,7 @@ compute_attribute_width(TargetEntry *tlistentry)
 static double
 relation_byte_size(double tuples, int width)
 {
-	return tuples * ((double) (width + sizeof(HeapTupleData)));
+	return tuples * ((double) MAXALIGN(width + sizeof(HeapTupleData)));
 }
 
 /*