Dispatch and discriminate REPACK/CLUSTER (T_RepackStmt) on PG19 (#8613)

[ihalatci]

Part of #8597 (PostgreSQL 19 support).

Addresses #8613 — dispatch and discriminate the PG19 unified REPACK/CLUSTER command (T_RepackStmt).

Note: the base branch of this PR is pg19-support (the PG19 integration branch), not main, so GitHub will not auto-close #8613 on merge — it is tracked manually.

What & why

PG19 removed ClusterStmt/T_ClusterStmt and replaced them with the unified RepackStmt, which backs three commands distinguished by RepackStmt.command: CLUSTER, the new REPACK, and VACUUM FULL. (VACUUM FULL still dispatches through T_VacuumStmt, so it never reaches this path.) The foundation PR (#8601) added a compile-only shim aliasing T_ClusterStmt → T_RepackStmt; that makes the names compile but is not correct behaviour, because CLUSTER and REPACK now collide on a single node tag and must be told apart.

This PR makes Citus dispatch and propagate the command correctly on PG19, while staying byte-for-byte regression-neutral on PG17/PG18.

Approach (Option A)

Citus propagates REPACK exactly like CLUSTER: the command is shipped to every shard placement through the existing CLUSTER code path. The worker name-relay (RelayEventExtendNames → AppendShardIdToName) mutates the parse tree in place, appending the shardId to both the target relation name and the index name (when USING INDEX is given), then ProcessUtilityParseTree executes the mutated node — there is no deparse-to-string step, so the existing relabel localizes REPACK's names with no new deparser work. CLUSTER vs REPACK are discriminated by RepackStmt.command, and the user-facing WARNING/ERROR wording is command-aware.

Files

• src/include/pg_version_compat.h — RepackStmt shim + version-portable helpers ClusterStmtIsRepack() / ClusterStmtCommandName() (compiled out on < PG19).
• src/backend/distributed/commands/cluster.c — PreprocessClusterStmt is command-aware (correct REPACK vs CLUSTER wording in the no-relation WARNING, the partitioned WARNING, and the VERBOSE ERROR).
• src/backend/distributed/commands/distribute_object_ops.c — GetDistributeObjectOps routes both CLUSTER and REPACK (shared T_ClusterStmt/T_RepackStmt tag).
• src/backend/distributed/relay/relay_event_utility.c — name-relay reads the RepackStmt layout (relation + index name).
• src/test/regress/sql/pg19.sql, expected/pg19.out, expected/pg19_0.out — new PG19-only acceptance test (+ the < PG19 early-quit variant).
• src/test/regress/multi_1_create_citus_schedule — registers the pg19 test.

Acceptance test (PG19-only, pg19.sql)

The test distributes repack_test (4 shards, replication factor 1) and demonstrates (not merely asserts) that distributed REPACK works:

• REPACK <t> USING INDEX <idx>, bare REPACK <t>, and CLUSTER <t> USING <idx> each rewrite every shard placement — the relfilenode changes on all shards (verified via run_command_on_shards).
• Distribution-invariance after the rewrites — proves REPACK changes storage, not distribution semantics:
  • placements_unchanged — shard set + placement nodes identical before/after (symmetric EXCEPT).
  • shard_count_unchanged — shard count identical before/after.
  • shard_row_mapping_unchanged — per-shard count(*) identical before/after (no row crossed a shard boundary).
  • distribution_unchanged — (partmethod, partkey, colocationid) intact.
  • routing still works — router queries a = 42 → (42, 2) and a = 100 → (100, 0); cross-shard aggregate count = 100, sum = 5050.
• Command-aware messaging — VERBOSE → REPACK/CLUSTER-worded ERROR; partitioned distributed table → not propagating REPACK command for partitioned table to worker nodes WARNING.

The < PG19 early-quit path keeps PG17/PG18 on the unchanged pg19_0.out, so this test is invisible to older majors.

Validation gate (WSL, pgenv, full -Werror, no demotion)

Version	Build (-Werror)	Regress
PG17.10	clean	4/4, no diffs (pg19 early-\q → unchanged pg19_0.out)
PG18.4	clean	4/4, no diffs (pg19 early-\q → unchanged pg19_0.out)
PG19beta1	clean	4/4 — distributed-REPACK acceptance test green (all invariance assertions t)

CLUSTER's existing tests/expected (multi_index_statements, pg15) are unchanged on all versions.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (pg19-support@1ef844c). Learn more about missing BASE report.

Additional details and impacted files

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