Tweak planner to use a minimum size estimate of 10 pages for a

never-yet-vacuumed relation.  This restores the pre-8.0 behavior of
avoiding seqscans during initial data loading, while still allowing
reasonable optimization after a table has been vacuumed.  Several
regression test cases revert to 7.4-like behavior, which is probably
a good sign.  Per gripes from Keith Browne and others.

src/backend/optimizer/util/plancat.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/util/plancat.c,v 1.100 2004/12/31 22:00:23 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/util/plancat.c,v 1.101 2005/03/24 19:14:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -234,7 +234,32 @@ estimate_rel_size(Relation rel, int32 *attr_widths,
 		case RELKIND_INDEX:
 		case RELKIND_TOASTVALUE:
 			/* it has storage, ok to call the smgr */
-			*pages = curpages = RelationGetNumberOfBlocks(rel);
+			curpages = RelationGetNumberOfBlocks(rel);
+
+			/*
+			 * HACK: if the relation has never yet been vacuumed, use a
+			 * minimum estimate of 10 pages.  This emulates a desirable
+			 * aspect of pre-8.0 behavior, which is that we wouldn't assume
+			 * a newly created relation is really small, which saves us from
+			 * making really bad plans during initial data loading.  (The
+			 * plans are not wrong when they are made, but if they are cached
+			 * and used again after the table has grown a lot, they are bad.)
+			 * It would be better to force replanning if the table size has
+			 * changed a lot since the plan was made ... but we don't
+			 * currently have any infrastructure for redoing cached plans at
+			 * all, so we have to kluge things here instead.
+			 *
+			 * We approximate "never vacuumed" by "has relpages = 0", which
+			 * means this will also fire on genuinely empty relations.  Not
+			 * great, but fortunately that's a seldom-seen case in the real
+			 * world, and it shouldn't degrade the quality of the plan too
+			 * much anyway to err in this direction.
+			 */
+			if (curpages < 10 && rel->rd_rel->relpages == 0)
+				curpages = 10;
+
+			/* report estimated # pages */
+			*pages = curpages;
 			/* quick exit if rel is clearly empty */
 			if (curpages == 0)
 			{

src/test/regress/expected/join.out

@@ -214,13 +214,13 @@ SELECT '' AS "xxx", t1.a, t2.e
   WHERE t1.a = t2.d;
  xxx | a | e  
 -----+---+----
+     | 0 |   
      | 1 | -1
      | 2 |  2
-     | 3 | -3
      | 2 |  4
+     | 3 | -3
      | 5 | -5
      | 5 | -5
-     | 0 |   
 (7 rows)
 
 --
@@ -1567,13 +1567,13 @@ SELECT '' AS "xxx", *
   FROM J1_TBL INNER JOIN J2_TBL USING (i);
  xxx | i | j |   t   | k  
 -----+---+---+-------+----
+     | 0 |   | zero  |   
      | 1 | 4 | one   | -1
-     | 2 | 3 | two   |  4
      | 2 | 3 | two   |  2
+     | 2 | 3 | two   |  4
      | 3 | 2 | three | -3
      | 5 | 0 | five  | -5
      | 5 | 0 | five  | -5
-     | 0 |   | zero  |   
 (7 rows)
 
 -- Same as above, slightly different syntax
@@ -1581,13 +1581,13 @@ SELECT '' AS "xxx", *
   FROM J1_TBL JOIN J2_TBL USING (i);
  xxx | i | j |   t   | k  
 -----+---+---+-------+----
+     | 0 |   | zero  |   
      | 1 | 4 | one   | -1
-     | 2 | 3 | two   |  4
      | 2 | 3 | two   |  2
+     | 2 | 3 | two   |  4
      | 3 | 2 | three | -3
      | 5 | 0 | five  | -5
      | 5 | 0 | five  | -5
-     | 0 |   | zero  |   
 (7 rows)
 
 SELECT '' AS "xxx", *
@@ -1623,35 +1623,35 @@ SELECT '' AS "xxx", *
   FROM J1_TBL NATURAL JOIN J2_TBL;
  xxx | i | j |   t   | k  
 -----+---+---+-------+----
+     | 0 |   | zero  |   
      | 1 | 4 | one   | -1
-     | 2 | 3 | two   |  4
      | 2 | 3 | two   |  2
+     | 2 | 3 | two   |  4
      | 3 | 2 | three | -3
      | 5 | 0 | five  | -5
      | 5 | 0 | five  | -5
-     | 0 |   | zero  |   
 (7 rows)
 
 SELECT '' AS "xxx", *
   FROM J1_TBL t1 (a, b, c) NATURAL JOIN J2_TBL t2 (a, d);
  xxx | a | b |   c   | d  
 -----+---+---+-------+----
+     | 0 |   | zero  |   
      | 1 | 4 | one   | -1
-     | 2 | 3 | two   |  4
      | 2 | 3 | two   |  2
+     | 2 | 3 | two   |  4
      | 3 | 2 | three | -3
      | 5 | 0 | five  | -5
      | 5 | 0 | five  | -5
-     | 0 |   | zero  |   
 (7 rows)
 
 SELECT '' AS "xxx", *
   FROM J1_TBL t1 (a, b, c) NATURAL JOIN J2_TBL t2 (d, a);
  xxx | a | b |  c   | d 
 -----+---+---+------+---
+     | 0 |   | zero |  
      | 2 | 3 | two  | 2
      | 4 | 1 | four | 2
-     | 0 |   | zero |  
 (3 rows)
 
 -- mismatch number of columns
@@ -1660,13 +1660,13 @@ SELECT '' AS "xxx", *
   FROM J1_TBL t1 (a, b) NATURAL JOIN J2_TBL t2 (a);
  xxx | a | b |   t   | k  
 -----+---+---+-------+----
+     | 0 |   | zero  |   
      | 1 | 4 | one   | -1
-     | 2 | 3 | two   |  4
      | 2 | 3 | two   |  2
+     | 2 | 3 | two   |  4
      | 3 | 2 | three | -3
      | 5 | 0 | five  | -5
      | 5 | 0 | five  | -5
-     | 0 |   | zero  |   
 (7 rows)
 
 --
@@ -1676,22 +1676,22 @@ SELECT '' AS "xxx", *
   FROM J1_TBL JOIN J2_TBL ON (J1_TBL.i = J2_TBL.i);
  xxx | i | j |   t   | i | k  
 -----+---+---+-------+---+----
+     | 0 |   | zero  | 0 |   
      | 1 | 4 | one   | 1 | -1
-     | 2 | 3 | two   | 2 |  4
      | 2 | 3 | two   | 2 |  2
+     | 2 | 3 | two   | 2 |  4
      | 3 | 2 | three | 3 | -3
      | 5 | 0 | five  | 5 | -5
      | 5 | 0 | five  | 5 | -5
-     | 0 |   | zero  | 0 |   
 (7 rows)
 
 SELECT '' AS "xxx", *
   FROM J1_TBL JOIN J2_TBL ON (J1_TBL.i = J2_TBL.k);
  xxx | i | j |  t   | i | k 
 -----+---+---+------+---+---
+     | 0 |   | zero |   | 0
      | 2 | 3 | two  | 2 | 2
      | 4 | 1 | four | 2 | 4
-     | 0 |   | zero |   | 0
 (3 rows)
 
 --
@@ -1760,13 +1760,13 @@ SELECT '' AS "xxx", *
   FROM J1_TBL RIGHT OUTER JOIN J2_TBL USING (i);
  xxx | i | j |   t   | k  
 -----+---+---+-------+----
+     | 0 |   | zero  |   
      | 1 | 4 | one   | -1
      | 2 | 3 | two   |  2
-     | 3 | 2 | three | -3
      | 2 | 3 | two   |  4
+     | 3 | 2 | three | -3
      | 5 | 0 | five  | -5
      | 5 | 0 | five  | -5
-     | 0 |   | zero  |   
      |   |   |       |   
      |   |   |       |  0
 (9 rows)
@@ -1775,13 +1775,13 @@ SELECT '' AS "xxx", *
   FROM J1_TBL RIGHT JOIN J2_TBL USING (i);
  xxx | i | j |   t   | k  
 -----+---+---+-------+----
+     | 0 |   | zero  |   
      | 1 | 4 | one   | -1
      | 2 | 3 | two   |  2
-     | 3 | 2 | three | -3
      | 2 | 3 | two   |  4
+     | 3 | 2 | three | -3
      | 5 | 0 | five  | -5
      | 5 | 0 | five  | -5
-     | 0 |   | zero  |   
      |   |   |       |   
      |   |   |       |  0
 (9 rows)

src/test/regress/expected/polymorphism.out

@@ -349,184 +349,184 @@ insert into t values(3,array[3],'b');
 select f3, myaggp01a(*) from t group by f3;
  f3 | myaggp01a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggp03a(*) from t group by f3;
  f3 | myaggp03a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggp03b(*) from t group by f3;
  f3 | myaggp03b 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggp05a(f1) from t group by f3;
  f3 | myaggp05a 
 ----+-----------
+ b  | {1,2,3}
  c  | {1,2}
  a  | {1,2,3}
- b  | {1,2,3}
 (3 rows)
 
 select f3, myaggp06a(f1) from t group by f3;
  f3 | myaggp06a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggp08a(f1) from t group by f3;
  f3 | myaggp08a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggp09a(f1) from t group by f3;
  f3 | myaggp09a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggp09b(f1) from t group by f3;
  f3 | myaggp09b 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggp10a(f1) from t group by f3;
  f3 | myaggp10a 
 ----+-----------
+ b  | {1,2,3}
  c  | {1,2}
  a  | {1,2,3}
- b  | {1,2,3}
 (3 rows)
 
 select f3, myaggp10b(f1) from t group by f3;
  f3 | myaggp10b 
 ----+-----------
+ b  | {1,2,3}
  c  | {1,2}
  a  | {1,2,3}
- b  | {1,2,3}
 (3 rows)
 
 select f3, myaggp20a(f1) from t group by f3;
  f3 | myaggp20a 
 ----+-----------
+ b  | {1,2,3}
  c  | {1,2}
  a  | {1,2,3}
- b  | {1,2,3}
 (3 rows)
 
 select f3, myaggp20b(f1) from t group by f3;
  f3 | myaggp20b 
 ----+-----------
+ b  | {1,2,3}
  c  | {1,2}
  a  | {1,2,3}
- b  | {1,2,3}
 (3 rows)
 
 select f3, myaggn01a(*) from t group by f3;
  f3 | myaggn01a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggn01b(*) from t group by f3;
  f3 | myaggn01b 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggn03a(*) from t group by f3;
  f3 | myaggn03a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggn05a(f1) from t group by f3;
  f3 | myaggn05a 
 ----+-----------
+ b  | {1,2,3}
  c  | {1,2}
  a  | {1,2,3}
- b  | {1,2,3}
 (3 rows)
 
 select f3, myaggn05b(f1) from t group by f3;
  f3 | myaggn05b 
 ----+-----------
+ b  | {1,2,3}
  c  | {1,2}
  a  | {1,2,3}
- b  | {1,2,3}
 (3 rows)
 
 select f3, myaggn06a(f1) from t group by f3;
  f3 | myaggn06a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggn06b(f1) from t group by f3;
  f3 | myaggn06b 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggn08a(f1) from t group by f3;
  f3 | myaggn08a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggn08b(f1) from t group by f3;
  f3 | myaggn08b 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggn09a(f1) from t group by f3;
  f3 | myaggn09a 
 ----+-----------
+ b  | {}
  c  | {}
  a  | {}
- b  | {}
 (3 rows)
 
 select f3, myaggn10a(f1) from t group by f3;
  f3 | myaggn10a 
 ----+-----------
+ b  | {1,2,3}
  c  | {1,2}
  a  | {1,2,3}
- b  | {1,2,3}
 (3 rows)
 

src/test/regress/expected/rules.out

@@ -1124,8 +1124,8 @@ insert into shoelace values ('sl10', 1000, 'magenta', 40.0, 'inch', 0.0);
 SELECT * FROM shoelace_obsolete;
   sl_name   | sl_avail |  sl_color  | sl_len | sl_unit  | sl_len_cm 
 ------------+----------+------------+--------+----------+-----------
- sl10       |     1000 | magenta    |     40 | inch     |     101.6
  sl9        |        0 | pink       |     35 | inch     |      88.9
+ sl10       |     1000 | magenta    |     40 | inch     |     101.6
 (2 rows)
 
 SELECT * FROM shoelace_candelete;