Revert "Avoid the creation of the free space map for small heap relations".

This feature was using a process local map to track the first few blocks
in the relation.  The map was reset each time we get the block with enough
freespace.  It was discussed that it would be better to track this map on
a per-relation basis in relcache and then invalidate the same whenever
vacuum frees up some space in the page or when FSM is created.  The new
design would be better both in terms of API design and performance.

List of commits reverted, in reverse chronological order:

06c8a5090e  Improve code comments in b0eaa4c51b.
13e8643bfc  During pg_upgrade, conditionally skip transfer of FSMs.
6f918159a9  Add more tests for FSM.
9c32e4c350  Clear the local map when not used.
29d108cdec  Update the documentation for FSM behavior..
08ecdfe7e5  Make FSM test portable.
b0eaa4c51b  Avoid creation of the free space map for small heap relations.

Discussion: https://postgr.es/m/20190416180452.3pm6uegx54iitbt5@alap3.anarazel.de

3 files changed, 51 insertions, 50 deletions

diff --git a/contrib/pageinspect/expected/page.out b/contrib/pageinspect/expected/page.out
index 74454801f5..3fcd9fbe6d 100644
--- a/contrib/pageinspect/expected/page.out
+++ b/contrib/pageinspect/expected/page.out
@@ -1,56 +1,48 @@
 CREATE EXTENSION pageinspect;
-CREATE TABLE test_rel_forks (a int);
--- Make sure there are enough blocks in the heap for the FSM to be created.
-INSERT INTO test_rel_forks SELECT i from generate_series(1,2000) i;
--- set up FSM and VM
-VACUUM test_rel_forks;
+CREATE TABLE test1 (a int, b int);
+INSERT INTO test1 VALUES (16777217, 131584);
+VACUUM test1;  -- set up FSM
 -- The page contents can vary, so just test that it can be read
 -- successfully, but don't keep the output.
-SELECT octet_length(get_raw_page('test_rel_forks', 'main', 0)) AS main_0;
+SELECT octet_length(get_raw_page('test1', 'main', 0)) AS main_0;
  main_0 
 --------
    8192
 (1 row)
 
-SELECT octet_length(get_raw_page('test_rel_forks', 'main', 100)) AS main_100;
-ERROR:  block number 100 is out of range for relation "test_rel_forks"
-SELECT octet_length(get_raw_page('test_rel_forks', 'fsm', 0)) AS fsm_0;
+SELECT octet_length(get_raw_page('test1', 'main', 1)) AS main_1;
+ERROR:  block number 1 is out of range for relation "test1"
+SELECT octet_length(get_raw_page('test1', 'fsm', 0)) AS fsm_0;
  fsm_0 
 -------
   8192
 (1 row)
 
-SELECT octet_length(get_raw_page('test_rel_forks', 'fsm', 20)) AS fsm_20;
-ERROR:  block number 20 is out of range for relation "test_rel_forks"
-SELECT octet_length(get_raw_page('test_rel_forks', 'vm', 0)) AS vm_0;
+SELECT octet_length(get_raw_page('test1', 'fsm', 1)) AS fsm_1;
+ fsm_1 
+-------
+  8192
+(1 row)
+
+SELECT octet_length(get_raw_page('test1', 'vm', 0)) AS vm_0;
  vm_0 
 ------
  8192
 (1 row)
 
-SELECT octet_length(get_raw_page('test_rel_forks', 'vm', 1)) AS vm_1;
-ERROR:  block number 1 is out of range for relation "test_rel_forks"
+SELECT octet_length(get_raw_page('test1', 'vm', 1)) AS vm_1;
+ERROR:  block number 1 is out of range for relation "test1"
 SELECT octet_length(get_raw_page('xxx', 'main', 0));
 ERROR:  relation "xxx" does not exist
-SELECT octet_length(get_raw_page('test_rel_forks', 'xxx', 0));
+SELECT octet_length(get_raw_page('test1', 'xxx', 0));
 ERROR:  invalid fork name
 HINT:  Valid fork names are "main", "fsm", "vm", and "init".
-EXPLAIN (costs off, analyze on, timing off, summary off) SELECT * FROM
-        fsm_page_contents(get_raw_page('test_rel_forks', 'fsm', 0));
-                         QUERY PLAN                         
-------------------------------------------------------------
- Function Scan on fsm_page_contents (actual rows=1 loops=1)
-(1 row)
-
-SELECT get_raw_page('test_rel_forks', 0) = get_raw_page('test_rel_forks', 'main', 0);
+SELECT get_raw_page('test1', 0) = get_raw_page('test1', 'main', 0);
  ?column? 
 ----------
  t
 (1 row)
 
-DROP TABLE test_rel_forks;
-CREATE TABLE test1 (a int, b int);
-INSERT INTO test1 VALUES (16777217, 131584);
 SELECT pagesize, version FROM page_header(get_raw_page('test1', 0));
  pagesize | version 
 ----------+---------
@@ -70,6 +62,26 @@ SELECT tuple_data_split('test1'::regclass, t_data, t_infomask, t_infomask2, t_bi
  {"\\x01000001","\\x00020200"}
 (1 row)
 
+SELECT * FROM fsm_page_contents(get_raw_page('test1', 'fsm', 0));
+ fsm_page_contents 
+-------------------
+ 0: 254           +
+ 1: 254           +
+ 3: 254           +
+ 7: 254           +
+ 15: 254          +
+ 31: 254          +
+ 63: 254          +
+ 127: 254         +
+ 255: 254         +
+ 511: 254         +
+ 1023: 254        +
+ 2047: 254        +
+ 4095: 254        +
+ fp_next_slot: 0  +
+ 
+(1 row)
+
 DROP TABLE test1;
 -- check that using any of these functions with a partitioned table or index
 -- would fail
diff --git a/contrib/pageinspect/sql/page.sql b/contrib/pageinspect/sql/page.sql
index e88598a48e..8ac9991837 100644
--- a/contrib/pageinspect/sql/page.sql
+++ b/contrib/pageinspect/sql/page.sql
@@ -1,36 +1,26 @@
 CREATE EXTENSION pageinspect;
 
-CREATE TABLE test_rel_forks (a int);
--- Make sure there are enough blocks in the heap for the FSM to be created.
-INSERT INTO test_rel_forks SELECT i from generate_series(1,2000) i;
+CREATE TABLE test1 (a int, b int);
+INSERT INTO test1 VALUES (16777217, 131584);
 
--- set up FSM and VM
-VACUUM test_rel_forks;
+VACUUM test1;  -- set up FSM
 
 -- The page contents can vary, so just test that it can be read
 -- successfully, but don't keep the output.
 
-SELECT octet_length(get_raw_page('test_rel_forks', 'main', 0)) AS main_0;
-SELECT octet_length(get_raw_page('test_rel_forks', 'main', 100)) AS main_100;
+SELECT octet_length(get_raw_page('test1', 'main', 0)) AS main_0;
+SELECT octet_length(get_raw_page('test1', 'main', 1)) AS main_1;
 
-SELECT octet_length(get_raw_page('test_rel_forks', 'fsm', 0)) AS fsm_0;
-SELECT octet_length(get_raw_page('test_rel_forks', 'fsm', 20)) AS fsm_20;
+SELECT octet_length(get_raw_page('test1', 'fsm', 0)) AS fsm_0;
+SELECT octet_length(get_raw_page('test1', 'fsm', 1)) AS fsm_1;
 
-SELECT octet_length(get_raw_page('test_rel_forks', 'vm', 0)) AS vm_0;
-SELECT octet_length(get_raw_page('test_rel_forks', 'vm', 1)) AS vm_1;
+SELECT octet_length(get_raw_page('test1', 'vm', 0)) AS vm_0;
+SELECT octet_length(get_raw_page('test1', 'vm', 1)) AS vm_1;
 
 SELECT octet_length(get_raw_page('xxx', 'main', 0));
-SELECT octet_length(get_raw_page('test_rel_forks', 'xxx', 0));
-
-EXPLAIN (costs off, analyze on, timing off, summary off) SELECT * FROM
-        fsm_page_contents(get_raw_page('test_rel_forks', 'fsm', 0));
-
-SELECT get_raw_page('test_rel_forks', 0) = get_raw_page('test_rel_forks', 'main', 0);
+SELECT octet_length(get_raw_page('test1', 'xxx', 0));
 
-DROP TABLE test_rel_forks;
-
-CREATE TABLE test1 (a int, b int);
-INSERT INTO test1 VALUES (16777217, 131584);
+SELECT get_raw_page('test1', 0) = get_raw_page('test1', 'main', 0);
 
 SELECT pagesize, version FROM page_header(get_raw_page('test1', 0));
 
@@ -39,6 +29,8 @@ SELECT page_checksum(get_raw_page('test1', 0), 0) IS NOT NULL AS silly_checksum_
 SELECT tuple_data_split('test1'::regclass, t_data, t_infomask, t_infomask2, t_bits)
     FROM heap_page_items(get_raw_page('test1', 0));
 
+SELECT * FROM fsm_page_contents(get_raw_page('test1', 'fsm', 0));
+
 DROP TABLE test1;
 
 -- check that using any of these functions with a partitioned table or index
diff --git a/contrib/pgstattuple/pgstatapprox.c b/contrib/pgstattuple/pgstatapprox.c
index ed62aef766..636c8d40ac 100644
--- a/contrib/pgstattuple/pgstatapprox.c
+++ b/contrib/pgstattuple/pgstatapprox.c
@@ -90,9 +90,6 @@ statapprox_heap(Relation rel, output_type *stat)
 		/*
 		 * If the page has only visible tuples, then we can find out the free
 		 * space from the FSM and move on.
-		 *
-		 * Note: If a relation has no FSM, GetRecordedFreeSpace() will report
-		 * zero free space.  This is fine for the purposes of approximation.
 		 */
 		if (VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
 		{