Teach TID store to skip bitmap for small numbers of offsets

The header portion of BlocktableEntry has enough padding space for
an array of 3 offsets (1 on 32-bit platforms). Use this space instead
of having a sparse bitmap array. This will take up a constant amount
of space no matter what the offsets are.

Reviewed (in an earlier version) by Masahiko Sawada

Discussion: https://postgr.es/m/CANWCAZYw+_KAaUNruhJfE=h6WgtBKeDG32St8vBJBEY82bGVRQ@mail.gmail.com
Discussion: https://postgr.es/m/CAD21AoBci3Hujzijubomo1tdwH3XtQ9F89cTNQ4bsQijOmqnEw@mail.gmail.com

2 files changed, 38 insertions, 0 deletions

diff --git a/src/test/modules/test_tidstore/expected/test_tidstore.out b/src/test/modules/test_tidstore/expected/test_tidstore.out
index 0ae2f970dac..06c610e84c2 100644
--- a/src/test/modules/test_tidstore/expected/test_tidstore.out
+++ b/src/test/modules/test_tidstore/expected/test_tidstore.out
@@ -36,6 +36,20 @@ SELECT do_set_block_offsets(blk, array_agg(off)::int2[])
     (VALUES (0), (1), (:maxblkno / 2), (:maxblkno - 1), (:maxblkno)) AS blocks(blk),
     (VALUES (1), (2), (:maxoffset / 2), (:maxoffset - 1), (:maxoffset)) AS offsets(off)
   GROUP BY blk;
+-- Test offsets embedded in the bitmap header.
+SELECT do_set_block_offsets(501, array[greatest((random() * :maxoffset)::int, 1)]::int2[]);
+ do_set_block_offsets 
+----------------------
+                  501
+(1 row)
+
+SELECT do_set_block_offsets(502, array_agg(DISTINCT greatest((random() * :maxoffset)::int, 1))::int2[])
+  FROM generate_series(1, 3);
+ do_set_block_offsets 
+----------------------
+                  502
+(1 row)
+
 -- Add enough TIDs to cause the store to appear "full", compared
 -- to the allocated memory it started out with. This is easier
 -- with memory contexts in local memory.
@@ -73,6 +87,20 @@ SELECT test_create(true);
  
 (1 row)
 
+-- Test offsets embedded in the bitmap header.
+SELECT do_set_block_offsets(501, array[greatest((random() * :maxoffset)::int, 1)]::int2[]);
+ do_set_block_offsets 
+----------------------
+                  501
+(1 row)
+
+SELECT do_set_block_offsets(502, array_agg(DISTINCT greatest((random() * :maxoffset)::int, 1))::int2[])
+  FROM generate_series(1, 3);
+ do_set_block_offsets 
+----------------------
+                  502
+(1 row)
+
 -- Random TIDs test. The offset numbers are randomized and must be
 -- unique and ordered.
 INSERT INTO hideblocks (blockno)
diff --git a/src/test/modules/test_tidstore/sql/test_tidstore.sql b/src/test/modules/test_tidstore/sql/test_tidstore.sql
index e5edfbb2649..bb31877b9a6 100644
--- a/src/test/modules/test_tidstore/sql/test_tidstore.sql
+++ b/src/test/modules/test_tidstore/sql/test_tidstore.sql
@@ -28,6 +28,11 @@ SELECT do_set_block_offsets(blk, array_agg(off)::int2[])
     (VALUES (1), (2), (:maxoffset / 2), (:maxoffset - 1), (:maxoffset)) AS offsets(off)
   GROUP BY blk;
 
+-- Test offsets embedded in the bitmap header.
+SELECT do_set_block_offsets(501, array[greatest((random() * :maxoffset)::int, 1)]::int2[]);
+SELECT do_set_block_offsets(502, array_agg(DISTINCT greatest((random() * :maxoffset)::int, 1))::int2[])
+  FROM generate_series(1, 3);
+
 -- Add enough TIDs to cause the store to appear "full", compared
 -- to the allocated memory it started out with. This is easier
 -- with memory contexts in local memory.
@@ -49,6 +54,11 @@ SELECT test_destroy();
 -- because unused static functions would raise warnings there.
 SELECT test_create(true);
 
+-- Test offsets embedded in the bitmap header.
+SELECT do_set_block_offsets(501, array[greatest((random() * :maxoffset)::int, 1)]::int2[]);
+SELECT do_set_block_offsets(502, array_agg(DISTINCT greatest((random() * :maxoffset)::int, 1))::int2[])
+  FROM generate_series(1, 3);
+
 -- Random TIDs test. The offset numbers are randomized and must be
 -- unique and ordered.
 INSERT INTO hideblocks (blockno)