[Query engine] Cross shard queries failing with unexpected field count in "D" message

[mijoharas]

When running a cross shard query of the format select count(1), user_id from example group by user_id; we get the error message:

unexpected field count in "D" message

Full reproduction details follow:

• first create a db

createdb pgdog_repro
createdb shard_repro_1
createdb shard_repro_2

• then connect and create a table

CREATE TABLE example (
  id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  user_id INTEGER
);

• let's throw some data in there

INSERT INTO example (user_id) VALUES (1), (2), (3), (1), (2), (3), (1);

• now, let's shard that into a couple of shards, gotta create them first

createdb shard_repro_1
createdb shard_repro_2

[[databases]]
name = "pgdog_repro"
host = "127.0.0.1"
port = 5432
role = "primary"

[[databases]]
name = "repro_sharded"
database_name = "shard_repro_1"
host = "127.0.0.1"
port = 5432
role = "primary"
shard = 0

[[databases]]
name = "repro_sharded"
database_name = "shard_repro_2"
host = "127.0.0.1"
port = 5432
role = "primary"
shard = 1

[[sharded_tables]]
database = "repro_sharded"
name = "example"
column = "user_id"
data_type = "bigint" # it's actually integer, but this should work the same

(ensure you have a user with schema_admin = true)

[[users]]
name = "whatever"
database = "repro_sharded"
schema_admin = true

[[users]]
name = "whatever"
database = "pgdog_repro"
schema_admin = true

• now, let's reshard everything

create publication sharding_test for table "example";

./target/release/pgdog -c ./local.pgdog.toml -u ./local.users.toml schema-sync --from-database pgdog_repro --to-database repro_sharded --publication sharding_test

# not specifying an option means sync then replicate
./target/release/pgdog -c ./local.pgdog.toml -u ./local.users.toml data-sync --from-database pgdog_repro --to-database repro_sharded --publication sharding_test --replication-slot sharding_test --sync-only

./target/release/pgdog -c ./local.pgdog.toml -u ./local.users.toml schema-sync --from-database pgdog_repro --to-database repro_sharded --publication sharding_test --data-sync-complete
./target/release/pgdog -c ./local.pgdog.toml -u ./local.users.toml schema-sync --from-database pgdog_repro --to-database repro_sharded --publication sharding_test --cutover

• then let's run pgdog

./target/release/pgdog -c ./local.pgdog.toml -u ./local.users.toml

• and connect to it

psql -h 127.0.0.1 -p 6432 repro_sharded

• and we've got our repro!

(michael)@127.0.0.1:6432 16:55:11 [repro_sharded]
# select count(1) from example;
 count
-------
     7
(1 row)

(michael)@127.0.0.1:6432 16:55:24 [repro_sharded]
# select count(1), user_id from example group by user_id;
unexpected field count in "D" message
(michael)@127.0.0.1:6432 16:55:33 [repro_sharded]
# select count(1), user_id from example where user_id in (1,3) group by user_id;
 count | user_id
-------+---------
     3 |       1
(1 row)

(michael)@127.0.0.1:6432 16:56:03 [repro_sharded]
# select count(1), user_id from example where user_id in (1,3) group by user_id;
 count | user_id
-------+---------
     2 |       3
(1 row)

[mijoharas]

Oh and almost forgot to mention, this was using latest master of pgdog 0ac6914038d016907f8eabf362c95dd19b6d933d