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website/content/blog/postgres-19-feature-preview-split-merge-partitions.md

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+---
+title: "Postgres 19 Feature Preview: ALTER TABLE SPLIT PARTITION and MERGE PARTITIONS"
+description: "Postgres 19 adds native DDL commands to split and merge partitions, replacing a risky multi-step manual workflow that has been a persistent operational pain point since declarative partitioning launched."
+date: "2026-02-18"
+---
+
+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.
+
+```sql
+-- 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.
+
+```sql
+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](https://www.postgresql.org/message-id/c73a1746-0cd0-6bdd-6b23-3ae0b7c0c582@postgrespro.ru) by Dmitry Koval at Postgres Professional. It was committed targeting PostgreSQL 17 in April 2024, then [reverted before the PG17 release](https://github.com/postgres/postgres/commit/3890d90c) 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 `DETACH`ing it, creating replacement tables, copying rows with `INSERT ... SELECT`, `ATTACH`ing 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.
+
+## References
+
+- [Commit f2e4cc42: Implement ALTER TABLE ... MERGE PARTITIONS ... command](https://github.com/postgres/postgres/commit/f2e4cc427951b7c46629fb7625a22f7898586f3a)
+- [Commit 4b3d1736: Implement ALTER TABLE ... SPLIT PARTITION ... command](https://github.com/postgres/postgres/commit/4b3d173629f4cd7ab6cd700d1053af5d5c7c9e37)
+- [pgsql-hackers: Add SPLIT PARTITION/MERGE PARTITIONS commands (original thread, May 2022)](https://www.postgresql.org/message-id/c73a1746-0cd0-6bdd-6b23-3ae0b7c0c582@postgrespro.ru)
+- [Revert commit 3890d90c: Revert support for ALTER TABLE ... MERGE/SPLIT PARTITION(S) commands](https://github.com/postgres/postgres/commit/3890d90c)