New cost model for planning, incorporating a penalty for random page

accesses versus sequential accesses, a (very crude) estimate of the
effects of caching on random page accesses, and cost to evaluate WHERE-
clause expressions.  Export critical parameters for this model as SET
variables.  Also, create SET variables for the planner's enable flags
(enable_seqscan, enable_indexscan, etc) so that these can be controlled
more conveniently than via PGOPTIONS.

Planner now estimates both startup cost (cost before retrieving
first tuple) and total cost of each path, so it can optimize queries
with LIMIT on a reasonable basis by interpolating between these costs.
Same facility is a win for EXISTS(...) subqueries and some other cases.

Redesign pathkey representation to achieve a major speedup in planning
(I saw as much as 5X on a 10-way join); also minor changes in planner
to reduce memory consumption by recycling discarded Path nodes and
not constructing unnecessary lists.

Minor cleanups to display more-plausible costs in some cases in
EXPLAIN output.

Initdb forced by change in interface to index cost estimation
functions.

src/test/regress/expected/alter_table.out

@@ -93,8 +93,11 @@ SELECT * FROM tmp;
 DROP TABLE tmp;
 --
 -- rename -
---   should preserve indices
+--   should preserve indices, which we can check by seeing if a SELECT
+--   chooses an indexscan; however, in the absence of vacuum statistics
+--   it might not.  Therefore, vacuum first.
 --
+VACUUM ANALYZE tenk1;
 ALTER TABLE tenk1 RENAME TO ten_k;
 -- 20 values, sorted 
 SELECT unique1 FROM ten_k WHERE unique1 < 20;

src/test/regress/expected/select.out

@@ -4,7 +4,8 @@
 -- btree index
 -- awk '{if($1<10){print;}else{next;}}' onek.data | sort +0n -1
 --
-SELECT onek.* WHERE onek.unique1 < 10;
+SELECT onek.* WHERE onek.unique1 < 10
+   ORDER BY onek.unique1;
  unique1 | unique2 | two | four | ten | twenty | hundred | thousand | twothousand | fivethous | tenthous | odd | even | stringu1 | stringu2 | string4 
 ---------+---------+-----+------+-----+--------+---------+----------+-------------+-----------+----------+-----+------+----------+----------+---------
        0 |     998 |   0 |    0 |   0 |      0 |       0 |        0 |           0 |         0 |        0 |   0 |    1 | AAAAAA   | KMBAAA   | OOOOxx

src/test/regress/sql/alter_table.sql

@@ -141,8 +141,12 @@ DROP TABLE tmp;
 
 --
 -- rename -
---   should preserve indices
+--   should preserve indices, which we can check by seeing if a SELECT
+--   chooses an indexscan; however, in the absence of vacuum statistics
+--   it might not.  Therefore, vacuum first.
 --
+VACUUM ANALYZE tenk1;
+
 ALTER TABLE tenk1 RENAME TO ten_k;
 
 -- 20 values, sorted 

src/test/regress/sql/select.sql

@@ -5,7 +5,8 @@
 -- btree index
 -- awk '{if($1<10){print;}else{next;}}' onek.data | sort +0n -1
 --
-SELECT onek.* WHERE onek.unique1 < 10;
+SELECT onek.* WHERE onek.unique1 < 10
+   ORDER BY onek.unique1;
 
 --
 -- awk '{if($1<20){print $1,$14;}else{next;}}' onek.data | sort +0nr -1