-- A write buffer that holds no batch (a transaction that inserted and rolled back, the only row of the buffer) has no batch array: the aggregate scan asks the buffer
-- for its batches all the same, and a copy of zero bytes from a NULL pointer is undefined (UBSan halts the backend and the server restarts: the sanitizer build of the full
-- tier fails on it, and so does every job that runs in the cluster while it recovers).
CREATE EXTENSION pg_sift;
SET max_parallel_workers_per_gather = 0;
SET sift.enable_agg = on;
SET enable_seqscan = off;
SET enable_indexscan = off;
SET enable_bitmapscan = off;
SET enable_indexonlyscan = off;
CREATE TABLE we (id int PRIMARY KEY, g int, v int) WITH (autovacuum_enabled = off);
INSERT INTO we SELECT i, i % 5, i FROM generate_series(1, 20000) i;
CREATE INDEX we_col ON we USING sift_col (id, g, v);
NOTICE:  sift_col index "we_col": 1 automatic choice (track_changes)
DETAIL:  track_changes = id
HINT:  Reasons, costs and how to turn each off: SELECT * FROM sift.index_choices('we_col');
BEGIN;
INSERT INTO we VALUES (500000, 1, 1);
ROLLBACK;
EXPLAIN (COSTS OFF) SELECT g, count(*) FROM we WHERE g < 3 GROUP BY g;
          QUERY PLAN           
-------------------------------
 Custom Scan (SiftAgg)
   Index: we_col
   Filters: g < 3
   Group Keys: g
   Pushed Aggregates: count(*)
(5 rows)

SELECT g, count(*), sum(v) FROM we WHERE g < 3 GROUP BY g ORDER BY g;
 g | count |   sum    
---+-------+----------
 0 |  4000 | 40010000
 1 |  4000 | 39994000
 2 |  4000 | 39998000
(3 rows)

SELECT count(*), sum(v) FROM we WHERE g = 4;
 count |   sum    
-------+----------
  4000 | 40006000
(1 row)

DROP TABLE we;
DROP EXTENSION pg_sift;