prefer-lz4-toast-compression

What triggers it

ALTER TABLE ... ALTER COLUMN ... SET COMPRESSION pglz (the AT_SetCompression subcommand with a pglz value), or SET default_toast_compression = 'pglz' as a VariableSetStmt — checked only when ctx.pgVersion is 14 or higher.

What does not

Any migration targeting PostgreSQL below 14 returns immediately, before the statement is inspected. SET COMPRESSION lz4 (or anything other than pglz) is skipped by the loop, as is SET default_toast_compression to anything other than 'pglz', and ALTER TABLE statements with no commands.

Where it applies

Applies to PostgreSQL 14 and later. It works on the SQL text alone — no database connection needed.

The lock, and what it blocks

None named — this is a metadata-only change. SET COMPRESSION doesn't rewrite existing TOASTed values, it only changes which algorithm is used for values written from here on, so there's nothing to block.

Why it matters

PostgreSQL 14 introduced lz4 as an alternative TOAST compression method. lz4 is 3-5x faster for both compression and decompression compared to pglz, with only slightly worse compression ratios.

Unsafe, and safe

ALTER TABLE users ALTER COLUMN bio SET COMPRESSION pglz;

ALTER TABLE users ALTER COLUMN bio SET COMPRESSION lz4;

What it assumes

Assumes the column actually stores TOAST-heavy data worth compressing — lz4 vs pglz makes no practical difference on a column that rarely stores values over the TOAST threshold, and the rule fires the same way regardless of column size or write volume.

What the CLI prints

⚠ [MP077] WARNING (line 1)
  Column "bio" on "users" uses pglz compression. Use lz4 on PG 14+ for 3-5x faster compression/decompression.

  Safe alternative:
  ALTER TABLE users ALTER COLUMN bio SET COMPRESSION lz4;

  Why: PostgreSQL 14 introduced lz4 as an alternative TOAST compression method. lz4 is 3-5x faster for both compression and decompression compared to the default pglz, with only slightly worse compression ratios. For JSONB, TEXT, and BYTEA columns with frequent reads/writes, lz4 significantly reduces CPU overhead.
  Docs: https://migrationpilot.dev/rules/mp077

Generated by running the CLI's own formatter over the flagged example above, so it is the text the tool actually produces. A real run also reports the other rules that fire on the same statement; those blocks are left out here.

Turning it off

For one statement, put a comment on the line before it:

-- migrationpilot-disable MP077
ALTER TABLE users ALTER COLUMN bio SET COMPRESSION pglz;

For the whole project, in .migrationpilotrc.yml — by name or by id:

rules:
  MP077: false

# or keep it, and downgrade it
rules:
  MP077:
    severity: warning

Try it

Open this rule's flagged example in the playground. It runs in your browser — edit it and watch the finding appear and disappear.