Not free (stripe) chunk buffers after de-serializing

Previously, we were only using chunk group reader for sequential scan.
However, to support index scans on columnar tables, now we use very
same low level functions for index scan too.

Since those low-level functions were only used for sequential scan, it
was guaranteed that we would never read the same chunk group more than
once, so we were freeing chunk buffers after deserializing them into a
separate buffer.

Now that we use those low level functions for index scan, we cannot
free chunk buffers since it's possible to read the same chunk group
again, such that:

- read chunk group 1 of stripe 5
- read chunk group 2 of stripe 5
- read chunk group 1 of stripe 5 again

Here, when we decide to read chunk group 1 for a second time,
chunk group 1 is not cached. Plus, before this commit, we were
freeing the chunk buffers for chunk group 1 after the first
read and then we were getting segfault or errors from low-level
de-compression APIs.

src/backend/columnar/columnar_reader.c

@@ -1378,13 +1378,6 @@ DeserializeChunkData(StripeBuffers *stripeBuffers, uint64 chunkIndex,
 								 chunkBuffers->valueCompressionType,
 								 chunkBuffers->decompressedValueSize);
 
-			if (chunkBuffers->valueCompressionType != COMPRESSION_NONE)
-			{
-				/* compressed data is not needed anymore */
-				pfree(chunkBuffers->valueBuffer->data);
-				pfree(chunkBuffers->valueBuffer);
-			}
-
 			DeserializeBoolArray(chunkBuffers->existsBuffer,
 								 chunkData->existsArray[columnIndex],
 								 rowCount);

src/test/regress/expected/columnar_indexes.out

@@ -533,5 +533,14 @@ WHERE
  t
 (1 row)
 
+CREATE TABLE revisit_same_cgroup(a INT, b TEXT) USING columnar;
+CREATE INDEX ON revisit_same_cgroup USING HASH (b);
+INSERT INTO revisit_same_cgroup SELECT random()*500, (random()*500)::INT::TEXT FROM generate_series(1, 100000) i;
+SELECT sum(a)>-1 FROM revisit_same_cgroup WHERE b = '1';
+ ?column?
+---------------------------------------------------------------------
+ t
+(1 row)
+
 SET client_min_messages TO WARNING;
 DROP SCHEMA columnar_indexes CASCADE;

src/test/regress/sql/columnar_indexes.sql

@@ -391,5 +391,11 @@ WHERE
   circle_col >= circle(point(7, 7), 350) AND
   float_col <= 5.0;
 
+CREATE TABLE revisit_same_cgroup(a INT, b TEXT) USING columnar;
+CREATE INDEX ON revisit_same_cgroup USING HASH (b);
+INSERT INTO revisit_same_cgroup SELECT random()*500, (random()*500)::INT::TEXT FROM generate_series(1, 100000) i;
+
+SELECT sum(a)>-1 FROM revisit_same_cgroup WHERE b = '1';
+
 SET client_min_messages TO WARNING;
 DROP SCHEMA columnar_indexes CASCADE;