[Bug]: EXPLAIN for UPDATE of compressed hypertable fails

[svenklemm]

EXPLAIN of UPDATE of compressed hypertable fails with bogus varno

BEGIN; EXPLAIN (analyze,costs off,timing off,summary off) UPDATE decompress_tracking SET value = value + 3 WHERE device = 'd1'; ROLLBACK;
BEGIN
Time: 1.739 ms
ERROR:  XX000: bogus varno: -3
LOCATION:  resolve_special_varno, ruleutils.c:7619

To reproduce:

CREATE TABLE decompress_tracking(time timestamptz not null, device text, value float);
SELECT table_name FROM create_hypertable('decompress_tracking','time');
ALTER TABLE decompress_tracking SET (timescaledb.compress, timescaledb.compress_segmentby='device');

INSERT INTO decompress_tracking SELECT '2020-01-01', 'd1', random() FROM generate_series(1,10);
INSERT INTO decompress_tracking SELECT '2020-01-01', 'd2', random() FROM generate_series(1,20);
INSERT INTO decompress_tracking SELECT '2020-01-01', 'd3', random() FROM generate_series(1,30);

SELECT count(compress_chunk(ch)) FROM show_chunks('decompress_tracking') ch;

BEGIN; UPDATE decompress_tracking SET value = value + 3 WHERE device = 'd1'; ROLLBACK; -- (or commit, doesn't make a difference)
BEGIN; EXPLAIN (analyze,costs off,timing off,summary off) UPDATE decompress_tracking SET value = value + 3 WHERE device = 'd1'; ROLLBACK;

[konskov]

it seems this was introduced by #5586

[konskov]

In the case of the explain plan, I think the issue is the fact that the plan we get from ExecInitNode/decompress_chunk_walker is a seqscan. Thus, in set_deparse_plan, dpns->index_tlist is set to NULL, but later in resolve_special_varno, we have var->varno == INDEX_VAR (because we have set it so in fix_index_qual) and yet dpns->index_tlist is NULL.
These observations make me think the issue is limited to the EXPLAIN, but given SDC#1350 https://github.com/timescale/Support-Dev-Collab/issues/1350, I am not so sure