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Transactional migration runners and PostgreSQL concurrent indexes are incompatible unless the runner explicitly opts out of the transaction.

Applicability and limits:
When a schema DSL can emit `CREATE INDEX CONCURRENTLY`, inspect the actual migration runner before accepting the generated SQL. PostgreSQL rejects concurrent index creation inside a transaction block. If the runner wraps the migration batch in one transaction and has no per-migration escape hatch, use a normal generated index or introduce an explicitly reviewed non-transactional deployment mechanism. Do not hand-edit generated migration artifacts when the repository requires schema-first generation. This is about migration execution compatibility; it does not mean a normal blocking index is always operationally acceptable for a very large table.

Method:
1. Inspect the generated SQL.
2. Inspect the installed migration runner implementation or repository wrapper to see whether it calls a transaction around migrations.
3. Regenerate from the schema authority after changing the index option.
4. Validate append-only journal history and run migration-validator tests.
5. Treat production table size and lock duration as a separate rollout decision.

Evidence/example — 2026-09-18:
In the Shield repository, Drizzle generated `CREATE INDEX CONCURRENTLY` from `.concurrently()`, while the installed PostgreSQL dialect migrator wraps all pending statements in `session.transaction(...)`. The generated migration was therefore invalid for the repository's staging/production migration path. Removing the schema-level concurrent option and regenerating produced a transaction-compatible partial index. The worktree journal validator and 13 migration-validator tests passed afterward.

Uncertainty:
A future Drizzle version or a custom runner may support non-transactional migrations; re-check the currently installed implementation rather than assuming this constraint forever.

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