fix(cross-shard): decode merged rows with the column types the shards return by murex971 · Pull Request #1917 · pgdogdev/pgdog

murex971 · 2026-10-29T14:04:01Z

Problem

Cross-shard merges (aggregates, ORDER BY, DISTINCT, LIMIT) decode rows using a RowDescription.
For named statements, that header came from the global prepared-statement cache. The cache is keyed
by SQL text only and is never refreshed, so after resharding widened integer keys to bigint,
PgDog decoded 8-byte values with a 4-byte header: FATAL 58000 type: wrong size slice. (Refer #1837)
Unnamed statements never had a header for Bind/Execute, so the decoder panicked:
decoder has no row description set. (Refer #1836)

Fix

Get the header from the shards with the rows, like the simple protocol already does:

When rows are merged across shards, PgDog sends Describe right before each Execute,
unless the client just described itself.
MultiShard uses those RowDescription/NoData replies to decode rows but doesn't forward them.
Remove the cached RowDescription from the global cache and the decoder's lookup of it,
plus the internal statement Describe for cross-shard variants, which is now redundant.

codecov · 2026-10-29T14:07:06Z

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

github-actions · 2026-10-29T14:35:04Z

Metal bench (arm64) failed. 4 of 103 measures outside bounds.

suite	throughput	cpu/txn	user cycles/txn	user instr/txn	kernel cycles/txn	ctx/txn
pooler		28.87 (+2.4% ± 1.4%), bound +2%	29,884 (+3.1% ± 2.3%), bound +2%	34,958 (-0.3% ± 0.04%)	39,403 (+2.2% ± 1.7%)
lb		29.92 (+2.5% ± 1.6%), bound +2%	31,402 (+3.2% ± 1.4%), bound +2%	35,824 (-0.3% ± 0.05%)	40,383 (+2.2% ± 2.3%)
sharding-w2		51.73 (-0.7%)	66,689 (-1.5%)	72,053 (-0.3%)	59,729 (-0.3%)	1.415 (+0.6%)
sharding-w8	157,144 (-1.2%)	46.5 (+0.4%)	66,803 (-0.7%)	55,885 (-0.3%)	53,030 (+1.7%)	0.1484 (+5.7%)

pooler: user cycles +3.1%, instructions flat. Unexplained.
lb: user cycles +3.2%, instructions flat. Unexplained.

Counters per txn

suite	frontend stall	l1i refill	backend stall
pooler	16,346 (+7.4% ± 4.9%)	3,478 (+3.4% ± 1.9%)	2,258 (-8.7% ± 2.9%)
lb	17,360 (+7.1% ± 2.8%)	3,574 (+3.4% ± 1.1%)	2,454 (-6.7% ± 2.7%)
sharding-w2	38,288 (-3.0%)	5,414 (-0.7%)	6,493 (+0.8%)
sharding-w8	37,919 (-3.0%)	5,039 (-1.9%)	9,166 (+8.2%)