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Postgres 19 Feature Preview: ALTER TABLE SPLIT PARTITION and MERGE PARTITIONS

February 18, 2026

Declarative partitioning has been in PostgreSQL since version 10. But once you defined your partition layout, you were mostly stuck with it. If a monthly range partition ballooned to 200GB and you needed to break it into weekly partitions, or if you wanted to consolidate a year's worth of daily partitions into a single archive partition, there was no DDL command for either operation — only a manual sequence of DETACH, INSERT ... SELECT, and ATTACH across multiple steps, with an ACCESS EXCLUSIVE lock on the parent table the entire time.

Postgres 19 fixes this with two new DDL commands: ALTER TABLE ... SPLIT PARTITION and ALTER TABLE ... MERGE PARTITIONS, committed by Alexander Korotkov on December 14, 2025.

Splitting a Partition

SPLIT PARTITION divides one existing partition into multiple new partitions. The original partition is replaced; any rows are redistributed into the new partitions automatically.

-- Before: one partition covering three regions
-- whatever_range_abc covers values ('a', 'b', 'c')

ALTER TABLE whatever_range
  SPLIT PARTITION whatever_range_abc INTO (
    PARTITION whatever_range_ab FOR VALUES IN ('a', 'b'),
    PARTITION whatever_range_c  FOR VALUES IN ('c')
  );

The typical scenario is a range partition that has grown too large. If your events_2024_q1 partition is overwhelming its tablespace or dominating autovacuum, you can split it into monthly partitions without touching any other part of the table.

Merging Partitions

MERGE PARTITIONS is the inverse: it combines multiple partitions into one. This is useful for archival workflows where you want to consolidate granular partitions into a coarser one, such as collapsing 365 daily partitions into a single yearly archive.

ALTER TABLE whatever_range
  MERGE PARTITIONS (whatever_range_c, whatever_range_de)
  INTO whatever_range_cde;

Both commands work for RANGE and LIST partitioning. Hash-partitioned tables are not supported.

Why This Took Three Years

The feature was first proposed in May 2022 by Dmitry Koval at Postgres Professional. It was committed targeting PostgreSQL 17 in April 2024, then reverted before the PG17 release after security issues were surfaced by Noah Misch and Robert Haas. It didn't make PG18 either. The December 2025 commit is the third attempt — this time it stuck.

What the Previous Workaround Looked Like

Without these commands, restructuring a partition meant manually DETACHing it, creating replacement tables, copying rows with INSERT ... SELECT, ATTACHing each new partition, then dropping the old one — all while coordinating two separate ACCESS EXCLUSIVE lock windows on the parent. Getting a boundary wrong left data in limbo with no rollback path.

Closing Thoughts

SPLIT PARTITION and MERGE PARTITIONS close a long-standing gap in PostgreSQL's partitioning story. The manual DETACH/INSERT/ATTACH workaround worked, but it was fragile, slow, and required careful coordination around locks. Having DDL commands for these operations means partition restructuring is now a single statement, visible in pg_dump, and consistent with how every other schema change works in PostgreSQL. One caveat: the current implementation holds ACCESS EXCLUSIVE for the entire operation including data movement, so it is not suited for large, actively written partitions — the commit message flags this as a foundation for future work with reduced locking.

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