fix: return an error for INSERT .. SELECT into sharded tables when router can't find shards of rows by jkaczman · Pull Request #1987 · pgdogdev/pgdog

jkaczman · 2026-11-06T18:38:53Z

Previously, when the subquery rewrite was off, queries matching INSERT INTO <sharded table> .. SELECT .. were routed incorrectly if the sharding key wasn't explicitly specified (e.g, from a generate_series or unnest function), as the router can't find the sharding keys for the rows belonging to the SELECT.

E.g., if we have a statement such as

INSERT INTO sharded (id, value) SELECT g, 'a' FROM generate_series(1, 10) g

the router, right now, has no idea what generate_series(1, 10) would return, thus, currently uses round-robin to route that query, meaning that, in a case where we have two shards, all the 10 rows from that generate_series function are routed to a single shard (e.g., Shard A), even though some of those ids belong on Shard B.

Now, the router returns an error for these kind of INSERTs in this situation.

A lot of this is irrelevant after subqueries are enabled by default, but until then, this prevents incorrect functionality stemming from this kind of query.

github-actions · 2026-11-09T21:43:04Z

Metal bench (arm64) passed. All 475 measures within bounds.

suite	throughput	cpu/txn	user cycles/txn	user instr/txn	kernel cycles/txn	kernel instr/txn	ctx/txn
pooler		18.56 (-0.2% ± 0.5%)	17,719 (-0.5% ± 0.1%)	31,989 (+0.0% ± 0.04%)	29,570 (-0.1% ± 1.4%)	51,203 (-0.1% ± 0.5%)
lb		19.22 (-0.1% ± 1.0%)	18,310 (-0.2% ± 0.1%)	32,884 (+0.0% ± 0.07%)	30,571 (+0.1% ± 1.7%)	52,378 (+0.3% ± 0.4%)
sharding-w2		33.69 (+1.3% ± 1.7%)	45,517 (+0.6% ± 1.5%)	97,319 (+0.0% ± 0.02%)	42,212 (+1.4% ± 2.3%)	53,618 (+0.1% ± 0.3%)	1.543 (+0.3% ± 0.5%)
sharding-w8	241,661 (+0.6% ± 3.1%)	29.77 (-0.8% ± 2.8%)	45,664 (+0.4% ± 1.2%)	52,881 (+0.0% ± 0.3%)	36,614 (-2.3% ± 5.0%)	42,343 (-0.0% ± 0.2%)	0.164 (+1.3% ± 2.3%)

Counters per txn

suite	frontend stall	l1i refill
pooler	7,025 (-0.3% ± 0.3%)	2,556 (-0.0% ± 0.1%)
lb	7,359 (+0.7% ± 0.6%)	2,608 (+0.1% ± 0.2%)
sharding-w2	19,370 (+0.9% ± 3.3%)	4,186 (-0.4% ± 2.0%)
sharding-w8	21,656 (+2.1% ± 2.2%)	4,254 (+1.3% ± 2.3%)

Microbench: all 462 measures within bounds.

Hot code per txn

path	base	head	change
pooler	110.1 KB	109.4 KB	-0.7 KB
sharding-w2	167.1 KB	166.2 KB	-0.9 KB

pooler:

crate	base	head	change
pgdog::frontend::client::query_engine	19.8 KB	20.1 KB	+0.2 KB
tokio	18.1 KB	18.1 KB	+0.0 KB
pgdog::backend::pool	17.2 KB	17.2 KB	+0.1 KB
aws-lc (TLS crypto)	9.9 KB	10.2 KB	+0.2 KB
rustls (TLS)	8.4 KB	8.4 KB	+0.0 KB
pgdog::frontend (other)	7.6 KB	7.6 KB	-0.1 KB
pgdog::backend::server	7.2 KB	6.9 KB	-0.3 KB
other	6.1 KB	5.7 KB	-0.4 KB
everything else	15.8 KB	15.2 KB	-0.5 KB

function	head	change
`pgdog::frontend::client::query_engine::query::<impl pgdog::frontend::client::query_engine:`	5.5 KB	+0.1 KB
`pgdog::frontend::client::Client::run::{{closure}}`	5.2 KB	+0.0 KB
`pgdog::frontend::client::query_engine::QueryEngine::handle::{{closure}}`	4.2 KB	+0.0 KB
`tokio::runtime::task::raw::poll`	3.1 KB	+0.0 KB
`pgdog::frontend::client::query_engine::query::<impl pgdog::frontend::client::query_engine:`	3.0 KB	+0.1 KB

sharding-w2:

crate	base	head	change
pgdog::frontend::router	26.8 KB	27.0 KB	+0.2 KB
pgdog::frontend::client::query_engine	23.0 KB	22.8 KB	-0.2 KB
tokio	19.4 KB	19.2 KB	-0.2 KB
pgdog::backend::pool	18.0 KB	18.0 KB	+0.0 KB
aws-lc (TLS crypto)	12.8 KB	12.8 KB	-0.1 KB
other	10.0 KB	10.0 KB	+0.0 KB
pgdog::frontend (other)	8.5 KB	8.8 KB	+0.2 KB
rustls (TLS)	8.2 KB	8.2 KB	-0.1 KB
everything else	40.3 KB	39.4 KB	-0.9 KB

function	head	change
`pgdog::frontend::client::Client::run::{{closure}}`	6.1 KB	+0.1 KB
`pgdog::frontend::client::query_engine::query::<impl pgdog::frontend::client::query_engine:`	5.2 KB	-0.1 KB
`pgdog::frontend::client::query_engine::QueryEngine::handle::{{closure}}`	4.2 KB	-0.1 KB
`pgdog::frontend::router::parser::query::select::<impl pgdog::frontend::router::parser::que`	3.4 KB	+0.0 KB
`tokio::runtime::task::raw::poll`	3.3 KB	-0.1 KB

lb flamegraph vs ba27030

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microbench flamegraph vs ba27030

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pooler flamegraph vs ba27030

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sharding-w2 flamegraph vs ba27030

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sharding-w8 flamegraph vs ba27030

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Logs

lb, microbench, pooler, sharding-w2, sharding-w8, verdict.json

Compared 0b7e835 with ba27030 on c8g.metal-24xl (on-demand).

github-actions · 2026-11-09T21:44:13Z

Metal bench (x86) passed. All 475 measures within bounds.

suite	throughput	cpu/txn	user cycles/txn	user instr/txn	kernel cycles/txn	kernel instr/txn	ctx/txn
pooler		34.86 (+0.3% ± 0.2%)	23,150 (+1.7% ± 0.8%)	33,858 (-0.0% ± 0.02%)	58,033 (-0.3% ± 0.3%)	38,562 (+0.1% ± 0.2%)
lb		35.89 (+0.4% ± 0.3%)	24,286 (+1.5% ± 0.6%)	34,803 (-0.0% ± 0.03%)	59,073 (+0.0% ± 0.5%)	39,216 (-0.1% ± 0.2%)
sharding-w2		56.94 (+0.3% ± 1.1%)	53,819 (+0.8% ± 1.4%)	101,187 (+0.0% ± 0.01%)	73,372 (+0.0% ± 1.1%)	43,023 (-0.0% ± 0.2%)	1.391 (-0.1% ± 0.5%)
sharding-w8	147,644 (-0.6% ± 0.6%)	48.38 (-0.0% ± 0.05%)	57,848 (-0.2% ± 0.4%)	56,666 (+0.0% ± 0.03%)	65,003 (+0.1% ± 0.5%)	34,218 (+0.0% ± 0.03%)	0.1661 (-1.2% ± 2.7%)

Counters per txn

suite	frontend stall	l1i refill
pooler	75,053 (+3.6% ± 1.0%)	3,629 (+0.9% ± 0.3%)
lb	78,139 (+3.4% ± 0.9%)	3,763 (+1.1% ± 0.1%)
sharding-w2	161,340 (+2.2% ± 2.9%)	6,154 (+2.2% ± 0.6%)
sharding-w8	124,381 (+0.1% ± 2.5%)	4,910 (+2.2% ± 0.2%)

Microbench: all 462 measures within bounds.

Microbench latency: 53 benches changed more than 3%

benchmark	latency	baseline	change	instructions change
simple_cold::set	2,648 ns	2,446 ns	+8.26% ± 1.61%	+0.00% ± 0.00%
simple_cold::delete_all_shards	2,969 ns	2,773 ns	+7.07% ± 0.97%	+0.01% ± 0.00%
extended_evicting::delete_all_shards	3,283 ns	3,067 ns	+7.03% ± 1.04%	+0.01% ± 0.00%
extended_evicting::set	2,928 ns	2,740 ns	+6.88% ± 2.27%	+0.00% ± 0.00%
simple_cold::begin	2,082 ns	1,952 ns	+6.66% ± 0.80%	+0.00% ± 0.00%
extended_evicting::select_omni	4,065 ns	3,813 ns	+6.61% ± 0.84%	+0.00% ± 0.00%
extended_cold::set	2,770 ns	2,602 ns	+6.46% ± 1.68%	+0.00% ± 0.00%
extended_cold::delete_all_shards	3,113 ns	2,925 ns	+6.43% ± 0.79%	+0.01% ± 0.00%
extended_evicting::select_all_shards	4,455 ns	4,199 ns	+6.09% ± 1.80%	+0.00% ± 0.00%
extended_evicting::begin	2,348 ns	2,214 ns	+6.05% ± 1.44%	+0.00% ± 0.00%
extended_evicting::delete_by_key	4,240 ns	4,009 ns	+5.77% ± 1.57%	+0.00% ± 0.00%
simple_cold::select_omni	3,717 ns	3,517 ns	+5.69% ± 1.47%	+0.00% ± 0.00%
extended_cold::begin	2,197 ns	2,081 ns	+5.58% ± 0.84%	+0.00% ± 0.00%
simple_cold::update_all_shards	3,651 ns	3,459 ns	+5.56% ± 1.84%	+0.00% ± 0.00%
extended_cold::select_omni	3,845 ns	3,643 ns	+5.54% ± 0.85%	+0.00% ± 0.00%
extended_evicting::update_all_shards	3,936 ns	3,731 ns	+5.50% ± 1.73%	+0.00% ± 0.00%
simple_cold::select_all_shards	4,111 ns	3,897 ns	+5.50% ± 1.82%	+0.00% ± 0.00%
extended_evicting::distinct	4,727 ns	4,497 ns	+5.12% ± 1.51%	+0.00% ± 0.00%
extended_cold::delete_by_key	4,024 ns	3,830 ns	+5.07% ± 0.95%	+0.00% ± 0.00%
simple_cold::delete_by_key	3,900 ns	3,716 ns	+4.96% ± 1.82%	+0.00% ± 0.00%
extended_cold::select_all_shards	4,244 ns	4,046 ns	+4.89% ± 1.12%	+0.00% ± 0.00%
extended_cached::group_by	2,990 ns	2,852 ns	+4.84% ± 1.51%	+0.01% ± 0.00%
extended_evicting::select_by_key	5,302 ns	5,064 ns	+4.70% ± 1.02%	+0.00% ± 0.00%
extended_cold::update_all_shards	3,767 ns	3,601 ns	+4.61% ± 1.31%	+0.00% ± 0.00%
extended_evicting::order_limit_offset	8,628 ns	8,250 ns	+4.58% ± 1.03%	-0.02% ± 0.00%
extended_cached::order_limit_offset	2,858 ns	2,738 ns	+4.39% ± 1.10%	+0.01% ± 0.00%
extended_cached::set	1,533 ns	1,471 ns	+4.22% ± 1.42%	+0.00% ± 0.00%
extended_cached::select_all_shards	2,500 ns	2,400 ns	+4.17% ± 1.38%	+0.01% ± 0.00%
simple_cold::select_by_key	4,976 ns	4,778 ns	+4.15% ± 2.66%	+0.00% ± 0.00%
simple_cold::distinct	4,375 ns	4,204 ns	+4.07% ± 1.67%	+0.00% ± 0.00%
extended_evicting::lookup	5,904 ns	5,674 ns	+4.06% ± 1.09%	+0.00% ± 0.00%
extended_cached::aggregate	2,995 ns	2,881 ns	+3.96% ± 0.67%	+0.01% ± 0.00%
extended_cold::update_by_key	4,618 ns	4,446 ns	+3.87% ± 0.88%	+0.00% ± 0.00%
extended_evicting::update_by_key	4,782 ns	4,606 ns	+3.82% ± 1.08%	+0.00% ± 0.00%
simple_cold::cte	7,183 ns	6,927 ns	+3.70% ± 1.46%	+0.00% ± 0.00%
extended_evicting::cte	7,529 ns	7,263 ns	+3.66% ± 1.18%	+0.00% ± 0.00%
extended_cached::min_max_sum	3,367 ns	3,249 ns	+3.63% ± 0.85%	+0.00% ± 0.00%
extended_evicting::subquery	6,344 ns	6,124 ns	+3.59% ± 0.78%	+0.00% ± 0.00%
simple_cold::order_limit_offset	8,279 ns	7,995 ns	+3.55% ± 0.37%	-0.02% ± 0.00%
extended_cold::order_limit_offset	8,336 ns	8,051 ns	+3.54% ± 0.74%	-0.02% ± 0.00%

Hot code per txn

path	base	head	change
pooler	100.6 KB	102.0 KB	+1.4 KB
sharding-w2	158.2 KB	156.9 KB	-1.3 KB

pooler:

crate	base	head	change
pgdog::frontend::client::query_engine	19.8 KB	19.6 KB	-0.2 KB
tokio	16.9 KB	17.4 KB	+0.5 KB
pgdog::backend::pool	16.1 KB	16.4 KB	+0.2 KB
pgdog::frontend (other)	9.1 KB	9.1 KB	+0.1 KB
rustls (TLS)	7.0 KB	7.1 KB	+0.1 KB
aws-lc (TLS crypto)	6.6 KB	6.7 KB	+0.1 KB
pgdog::backend::server	6.3 KB	6.2 KB	-0.1 KB
other	5.5 KB	5.5 KB	+0.0 KB
everything else	13.3 KB	14.0 KB	+0.7 KB

function	head	change
`pgdog::frontend::client::Client::run::{{closure}}`	6.9 KB	+0.1 KB
`pgdog::frontend::client::query_engine::query::<impl pgdog::frontend::client::query_engine:`	5.7 KB	+0.1 KB
`pgdog::frontend::client::query_engine::QueryEngine::handle::{{closure}}`	4.9 KB	+0.2 KB
`tokio::runtime::task::raw::poll`	3.6 KB	+0.0 KB
`tokio_rustls::common::Stream<IO,C>::read_io`	2.8 KB	+0.1 KB

sharding-w2:

crate	base	head	change
pgdog::frontend::router	25.6 KB	24.3 KB	-1.3 KB
pgdog::frontend::client::query_engine	23.7 KB	23.2 KB	-0.5 KB
tokio	18.0 KB	17.9 KB	-0.1 KB
pgdog::backend::pool	18.1 KB	17.2 KB	-0.9 KB
aws-lc (TLS crypto)	10.1 KB	10.3 KB	+0.2 KB
other	10.2 KB	10.2 KB	-0.1 KB
pgdog::frontend (other)	10.1 KB	10.1 KB	+0.1 KB
pgdog::backend::server	7.4 KB	7.3 KB	-0.1 KB
everything else	35.1 KB	36.4 KB	+1.3 KB

function	head	change
`pgdog::frontend::client::Client::run::{{closure}}`	7.6 KB	+0.1 KB
`pgdog::frontend::client::query_engine::query::<impl pgdog::frontend::client::query_engine:`	5.6 KB	-0.2 KB
`pgdog::frontend::client::query_engine::QueryEngine::handle::{{closure}}`	4.9 KB	+0.2 KB
`pgdog::frontend::router::parser::query::select::<impl pgdog::frontend::router::parser::que`	4.2 KB	+0.1 KB
`tokio::runtime::task::raw::poll`	3.7 KB	-0.2 KB

lb flamegraph vs ba27030

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microbench flamegraph vs ba27030

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pooler flamegraph vs ba27030

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Open interactive.

sharding-w2 flamegraph vs ba27030

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Open interactive.

sharding-w8 flamegraph vs ba27030

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Open interactive.

Logs

lb, microbench, pooler, sharding-w2, sharding-w8, verdict.json

Compared 0b7e835 with ba27030 on m7a.metal-48xl (spot).

codecov · 2026-11-09T21:57:48Z

Codecov Report

❌ Patch coverage is 92.39130% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...og/src/frontend/client/query_engine/route_query.rs	0.00%	7 Missing ⚠️

📢 Thoughts on this report? Let us know!

…uter can't find shards of rows

jkaczman added this pull request to stack #1986 November 6, 2026 18:38

jkaczman mentioned this pull request Nov 9, 2026

feat: split INSERT .. SELECT <columns> FROM unnest(..) into sharded tables (not only SELECT *) #2000

Open

jkaczman force-pushed the jk-insert-select-shard-error branch from 3dee97a to f6c30c2 Compare November 9, 2026 21:34

jkaczman force-pushed the jk-insert-select-shard-error branch 3 times, most recently from 7d572f1 to 0b7e835 Compare November 9, 2026 21:55

Base automatically changed from jk-subqueries-plain-ctes to main November 9, 2026 23:50

fix: return an error for INSERT .. SELECT into sharded tables when ro…

6df7724

…uter can't find shards of rows

jkaczman force-pushed the jk-insert-select-shard-error branch from 0b7e835 to 6df7724 Compare November 9, 2026 23:50

jkaczman marked this pull request as ready for review November 9, 2026 23:50

jkaczman merged commit 87965fb into main Nov 9, 2026
41 checks passed

jkaczman deleted the jk-insert-select-shard-error branch November 9, 2026 23:52

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fix: return an error for INSERT .. SELECT into sharded tables when router can't find shards of rows - #1987

fix: return an error for INSERT .. SELECT into sharded tables when router can't find shards of rows#1987
jkaczman merged 1 commit into
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jk-insert-select-shard-error

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