Test if a table has a primary key in Postgres

primary-key-postgres.md

This returns the names and data types of all columns of the primary key for the tablename table:

FROM   pg_index i
JOIN   pg_attribute a ON a.attrelid = i.indrelid
                     AND a.attnum = ANY(i.indkey)
WHERE  i.indrelid = 'tablename'::regclass
AND    i.indisprimary;

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