Adding Columns and Constraints Without Downtime
Safe schema change comes down to one idea: don't make PostgreSQL rewrite the whole table while holding a heavy lock.
Either change only the catalog so the data is left untouched, or split the job into a quick lock followed by a slow scan that barely locks anything. That's how big changes ship without downtime.
Pro members see the rest of this lesson
- A 4-step state diagram: Add → Declare → Validate → Trust
- The full mechanism: what PostgreSQL does internally, in source terms
- 4 SQL queries you can run, each labelled by how it was verified
- The closing insight, the mistake it prevents, and what it changes in your work
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Check it against the source1 citation in postgres/postgres · file, symbol and line verified on REL_17_STABLE
Primary symbol: ATExecAddColumn · line 7021
Anchored to postgres/postgres on REL_17_STABLE and cross-checked against the manual for PostgreSQL 15–18. Every query here was run and its output captured on a throwaway PostgreSQL 17.10 lab; corrections are noted inline. §5.4 Constraints in the official docs →
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