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

src/test/regress/expected/fsm.out

@@ -1,73 +0,0 @@
---
--- Free Space Map test
---
-SELECT current_setting('block_size')::integer AS blocksize,
-current_setting('block_size')::integer / 8 AS strsize
-\gset
-CREATE TABLE fsm_check_size (num int, str text);
--- Fill 3 blocks with one record each
-ALTER TABLE fsm_check_size SET (fillfactor=15);
-INSERT INTO fsm_check_size SELECT i, rpad('', :strsize, 'a')
-FROM generate_series(1,3) i;
--- There should be no FSM
-VACUUM fsm_check_size;
-SELECT pg_relation_size('fsm_check_size', 'main') / :blocksize AS heap_nblocks,
-pg_relation_size('fsm_check_size', 'fsm') / :blocksize AS fsm_nblocks;
- heap_nblocks | fsm_nblocks 
---------------+-------------
-            3 |           0
-(1 row)
-
--- The following operations are for testing the functionality of the local
--- in-memory map. In particular, we want to be able to insert into some
--- other block than the one at the end of the heap, without using a FSM.
--- Fill most of the last block
-ALTER TABLE fsm_check_size SET (fillfactor=100);
-INSERT INTO fsm_check_size SELECT i, rpad('', :strsize, 'a')
-FROM generate_series(101,105) i;
--- Make sure records can go into any block but the last one
-ALTER TABLE fsm_check_size SET (fillfactor=30);
--- Insert large record and make sure it does not cause the relation to extend
-INSERT INTO fsm_check_size VALUES (111, rpad('', :strsize, 'a'));
-VACUUM fsm_check_size;
-SELECT pg_relation_size('fsm_check_size', 'main') / :blocksize AS heap_nblocks,
-pg_relation_size('fsm_check_size', 'fsm') / :blocksize AS fsm_nblocks;
- heap_nblocks | fsm_nblocks 
---------------+-------------
-            3 |           0
-(1 row)
-
--- Extend table with enough blocks to exceed the FSM threshold
-DO $$
-DECLARE curtid tid;
-num int;
-BEGIN
-num = 11;
-  LOOP
-    INSERT INTO fsm_check_size VALUES (num, 'b') RETURNING ctid INTO curtid;
-    EXIT WHEN curtid >= tid '(4, 0)';
-    num = num + 1;
-  END LOOP;
-END;
-$$;
-VACUUM fsm_check_size;
-SELECT pg_relation_size('fsm_check_size', 'fsm') / :blocksize AS fsm_nblocks;
- fsm_nblocks 
--------------
-           3
-(1 row)
-
--- Add long random string to extend TOAST table to 1 block
-INSERT INTO fsm_check_size
-VALUES(0, (SELECT string_agg(md5(chr(i)), '')
-		   FROM generate_series(1, :blocksize / 100) i));
-VACUUM fsm_check_size;
-SELECT pg_relation_size(reltoastrelid, 'main') / :blocksize AS toast_nblocks,
-pg_relation_size(reltoastrelid, 'fsm') / :blocksize AS toast_fsm_nblocks
-FROM pg_class WHERE relname = 'fsm_check_size';
- toast_nblocks | toast_fsm_nblocks 
----------------+-------------------
-             1 |                 0
-(1 row)
-
-DROP TABLE fsm_check_size;

src/test/regress/parallel_schedule

@@ -20,7 +20,7 @@ test: boolean char name varchar text int2 int4 int8 oid float4 float8 bit numeri
 # strings depends on char, varchar and text
 # numerology depends on int2, int4, int8, float4, float8
 # ----------
-test: strings numerology point lseg line box path polygon circle date time timetz timestamp timestamptz interval inet macaddr macaddr8 tstypes fsm
+test: strings numerology point lseg line box path polygon circle date time timetz timestamp timestamptz interval inet macaddr macaddr8 tstypes
 
 # ----------
 # Another group of parallel tests

src/test/regress/serial_schedule

@@ -40,7 +40,6 @@ test: inet
 test: macaddr
 test: macaddr8
 test: tstypes
-test: fsm
 test: geometry
 test: horology
 test: regex

src/test/regress/sql/fsm.sql

@@ -1,66 +0,0 @@
---
--- Free Space Map test
---
-SELECT current_setting('block_size')::integer AS blocksize,
-current_setting('block_size')::integer / 8 AS strsize
-\gset
-
-CREATE TABLE fsm_check_size (num int, str text);
-
--- Fill 3 blocks with one record each
-ALTER TABLE fsm_check_size SET (fillfactor=15);
-INSERT INTO fsm_check_size SELECT i, rpad('', :strsize, 'a')
-FROM generate_series(1,3) i;
-
--- There should be no FSM
-VACUUM fsm_check_size;
-SELECT pg_relation_size('fsm_check_size', 'main') / :blocksize AS heap_nblocks,
-pg_relation_size('fsm_check_size', 'fsm') / :blocksize AS fsm_nblocks;
-
--- The following operations are for testing the functionality of the local
--- in-memory map. In particular, we want to be able to insert into some
--- other block than the one at the end of the heap, without using a FSM.
-
--- Fill most of the last block
-ALTER TABLE fsm_check_size SET (fillfactor=100);
-INSERT INTO fsm_check_size SELECT i, rpad('', :strsize, 'a')
-FROM generate_series(101,105) i;
-
--- Make sure records can go into any block but the last one
-ALTER TABLE fsm_check_size SET (fillfactor=30);
-
--- Insert large record and make sure it does not cause the relation to extend
-INSERT INTO fsm_check_size VALUES (111, rpad('', :strsize, 'a'));
-
-VACUUM fsm_check_size;
-SELECT pg_relation_size('fsm_check_size', 'main') / :blocksize AS heap_nblocks,
-pg_relation_size('fsm_check_size', 'fsm') / :blocksize AS fsm_nblocks;
-
--- Extend table with enough blocks to exceed the FSM threshold
-DO $$
-DECLARE curtid tid;
-num int;
-BEGIN
-num = 11;
-  LOOP
-    INSERT INTO fsm_check_size VALUES (num, 'b') RETURNING ctid INTO curtid;
-    EXIT WHEN curtid >= tid '(4, 0)';
-    num = num + 1;
-  END LOOP;
-END;
-$$;
-
-VACUUM fsm_check_size;
-SELECT pg_relation_size('fsm_check_size', 'fsm') / :blocksize AS fsm_nblocks;
-
--- Add long random string to extend TOAST table to 1 block
-INSERT INTO fsm_check_size
-VALUES(0, (SELECT string_agg(md5(chr(i)), '')
-		   FROM generate_series(1, :blocksize / 100) i));
-
-VACUUM fsm_check_size;
-SELECT pg_relation_size(reltoastrelid, 'main') / :blocksize AS toast_nblocks,
-pg_relation_size(reltoastrelid, 'fsm') / :blocksize AS toast_fsm_nblocks
-FROM pg_class WHERE relname = 'fsm_check_size';
-
-DROP TABLE fsm_check_size;