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

Previously, all heaps had FSMs. For very small tables, this means that the
FSM took up more space than the heap did. This is wasteful, so now we
refrain from creating the FSM for heaps with 4 pages or fewer. If the last
known target block has insufficient space, we still try to insert into some
other page before giving up and extending the relation, since doing
otherwise leads to table bloat. Testing showed that trying every page
penalized performance slightly, so we compromise and try every other page.
This way, we visit at most two pages. Any pages with wasted free space
become visible at next relation extension, so we still control table bloat.
As a bonus, directly attempting one or two pages can even be faster than
consulting the FSM would have been.

Once the FSM is created for a heap we don't remove it even if somebody
deletes all the rows from the corresponding relation.  We don't think it is
a useful optimization as it is quite likely that relation will again grow
to the same size.

Author: John Naylor with design inputs and some code contribution by Amit Kapila
Reviewed-by: Amit Kapila
Tested-by: Mithun C Y
Discussion: https://www.postgresql.org/message-id/CAJVSVGWvB13PzpbLEecFuGFc5V2fsO736BsdTakPiPAcdMM5tQ@mail.gmail.com

src/test/regress/expected/fsm.out

@@ -0,0 +1,75 @@
+--
+-- Free Space Map test
+--
+CREATE TABLE fsm_check_size (num int, str text);
+-- Fill 3 blocks with as many large records as will fit
+-- No FSM
+INSERT INTO fsm_check_size SELECT i, rpad('', 1024, 'a')
+FROM generate_series(1,7*3) i;
+VACUUM fsm_check_size;
+SELECT pg_relation_size('fsm_check_size', 'main') AS heap_size,
+pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+ heap_size | fsm_size 
+-----------+----------
+     24576 |        0
+(1 row)
+
+-- Clear some space on block 0
+DELETE FROM fsm_check_size WHERE num <= 5;
+VACUUM fsm_check_size;
+-- Insert small record in block 2 to set the cached smgr targetBlock
+INSERT INTO fsm_check_size VALUES(99, 'b');
+-- Insert large record and make sure it goes in block 0 rather than
+-- causing the relation to extend
+INSERT INTO fsm_check_size VALUES (101, rpad('', 1024, 'a'));
+SELECT pg_relation_size('fsm_check_size', 'main') AS heap_size,
+pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+ heap_size | fsm_size 
+-----------+----------
+     24576 |        0
+(1 row)
+
+-- Extend table with enough blocks to exceed the FSM threshold
+-- FSM is created and extended to 3 blocks
+INSERT INTO fsm_check_size SELECT i, 'c' FROM generate_series(200,1200) i;
+VACUUM fsm_check_size;
+SELECT pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+ fsm_size 
+----------
+    24576
+(1 row)
+
+-- Truncate heap to 1 block
+-- No change in FSM
+DELETE FROM fsm_check_size WHERE num > 7;
+VACUUM fsm_check_size;
+SELECT pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+ fsm_size 
+----------
+    24576
+(1 row)
+
+-- Truncate heap to 0 blocks
+-- FSM now truncated to 2 blocks
+DELETE FROM fsm_check_size;
+VACUUM fsm_check_size;
+SELECT pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+ fsm_size 
+----------
+    16384
+(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(1,100) i));
+VACUUM fsm_check_size;
+SELECT pg_relation_size(reltoastrelid, 'main') AS toast_size,
+pg_relation_size(reltoastrelid, 'fsm') AS toast_fsm_size
+FROM pg_class WHERE relname = 'fsm_check_size';
+ toast_size | toast_fsm_size 
+------------+----------------
+       8192 |              0
+(1 row)
+
+DROP TABLE fsm_check_size;

src/test/regress/parallel_schedule

@@ -68,6 +68,12 @@ test: create_aggregate create_function_3 create_cast constraints triggers inheri
 # ----------
 test: sanity_check
 
+# ----------
+# fsm does a delete followed by vacuum, and running it in parallel can prevent
+# removal of rows.
+# ----------
+test: fsm
+
 # ----------
 # Believe it or not, select creates a table, subsequent
 # tests need.

src/test/regress/serial_schedule

@@ -80,6 +80,7 @@ test: roleattributes
 test: create_am
 test: hash_func
 test: sanity_check
+test: fsm
 test: errors
 test: select
 test: select_into

src/test/regress/sql/fsm.sql

@@ -0,0 +1,55 @@
+--
+-- Free Space Map test
+--
+
+CREATE TABLE fsm_check_size (num int, str text);
+
+-- Fill 3 blocks with as many large records as will fit
+-- No FSM
+INSERT INTO fsm_check_size SELECT i, rpad('', 1024, 'a')
+FROM generate_series(1,7*3) i;
+VACUUM fsm_check_size;
+SELECT pg_relation_size('fsm_check_size', 'main') AS heap_size,
+pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+
+-- Clear some space on block 0
+DELETE FROM fsm_check_size WHERE num <= 5;
+VACUUM fsm_check_size;
+
+-- Insert small record in block 2 to set the cached smgr targetBlock
+INSERT INTO fsm_check_size VALUES(99, 'b');
+
+-- Insert large record and make sure it goes in block 0 rather than
+-- causing the relation to extend
+INSERT INTO fsm_check_size VALUES (101, rpad('', 1024, 'a'));
+SELECT pg_relation_size('fsm_check_size', 'main') AS heap_size,
+pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+
+-- Extend table with enough blocks to exceed the FSM threshold
+-- FSM is created and extended to 3 blocks
+INSERT INTO fsm_check_size SELECT i, 'c' FROM generate_series(200,1200) i;
+VACUUM fsm_check_size;
+SELECT pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+
+-- Truncate heap to 1 block
+-- No change in FSM
+DELETE FROM fsm_check_size WHERE num > 7;
+VACUUM fsm_check_size;
+SELECT pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+
+-- Truncate heap to 0 blocks
+-- FSM now truncated to 2 blocks
+DELETE FROM fsm_check_size;
+VACUUM fsm_check_size;
+SELECT pg_relation_size('fsm_check_size', 'fsm') AS fsm_size;
+
+-- 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(1,100) i));
+VACUUM fsm_check_size;
+SELECT pg_relation_size(reltoastrelid, 'main') AS toast_size,
+pg_relation_size(reltoastrelid, 'fsm') AS toast_fsm_size
+FROM pg_class WHERE relname = 'fsm_check_size';
+
+DROP TABLE fsm_check_size;