Always reset expr context in DecompressChunk

[mkindahl]

When decompressing very large compressed chunks as part of executing a
DecompressChunk, the expression memory context was not reset after
each qual evaluation if the qual did not match, meaning that if the
qual did not match often, memory allocations would start to accumulate.

This commit fixes this by resetting the expression memory context for
each tuple when executing a DecompressChunk node, even if the qual
did not match.

Fixes #3620

[mkindahl]

Tested locally on a database with 12 million rows where not all match the conditions.

Before

Result of executing query from #3620 without patch

mats=# EXPLAIN ANALYZE SELECT * FROM events 
WHERE event->'timestamp' >= '0' 
AND event->'timestamp' < '1630454535290' 
AND event-> 'div' in ('"ABB"')
AND event->'mnemonic' = '"KQN_805WbKD_7_Z1Go4"';
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!?> 

After

Result of executing query from #3620 with patch

mats=# EXPLAIN ANALYZE SELECT * FROM events 
WHERE event->'timestamp' >= '0' 
AND event->'timestamp' < '1630454535290' 
AND event-> 'div' in ('"ABB"')
AND event->'mnemonic' = '"KQN_805WbKD_7_Z1Go4"';
                                                                                                              QUERY P
LAN                                                                                                               
---------------------------------------------------------------------------------------------------------------------
------------------------------------------------------------------------------------------------------------------
 Custom Scan (DecompressChunk) on _hyper_29_880_chunk  (cost=0.02..317.02 rows=13002000 width=522) (actual time=16724
.921..31391.073 rows=1084133 loops=1)
   Filter: (((event -> 'timestamp'::text) >= '0'::jsonb) AND ((event -> 'timestamp'::text) < '1630454535290'::jsonb) 
AND ((event -> 'div'::text) = '"ABB"'::jsonb) AND ((event -> 'mnemonic'::text) = '"KQN_805WbKD_7_Z1Go4"'::jsonb))
   Rows Removed by Filter: 11915867
   ->  Seq Scan on compress_hyper_30_881_chunk  (cost=0.00..317.02 rows=13002 width=109) (actual time=0.190..152.605 
rows=13002 loops=1)
 Planning Time: 24.489 ms
 Execution Time: 43933.637 ms
(6 rows)

[codecov]

Codecov Report

Merging #3747 (1646de9) into master (0fecefd) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3747      +/-   ##
==========================================
- Coverage   90.29%   90.29%   -0.01%     
==========================================
  Files         213      213              
  Lines       37655    37654       -1     
==========================================
- Hits        34001    33999       -2     
- Misses       3654     3655       +1     

Impacted Files	Coverage Δ
tsl/src/chunk_api.c	94.88% <100.00%> (ø)
tsl/src/nodes/decompress_chunk/exec.c	96.36% <100.00%> (ø)
tsl/src/nodes/data_node_dispatch.c	97.10% <0.00%> (-0.25%)	⬇️
tsl/src/bgw_policy/job.c	87.50% <0.00%> (-0.06%)	⬇️

---

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[erimatnor]

Wish we could have those screenshots auto-generated for every PR :)