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في هذا الدرس

see how a schema change can block production.

الموضوعات المخطط لها

• Many ALTER TABLE forms take an ACCESS EXCLUSIVE lock, which blocks all reads and writes on the table.
• A waiting DDL statement also blocks later queries queued behind it, so one slow transaction can cause a pile-up.
• Table rewrites (changing a column type, some defaults on older versions) hold the lock while copying data.
• Old and new application versions run together during a deploy, so the schema must work for both.