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Simplify and speed up Postgres query for primary_keys
primary_keys(table) needs to query various metadata tables in Postgres to determine the primary key for the table. Previously, it did so using a complex common table expression against pg_constraint and pg_attribute. This patch simplifies the query by joining pg_index against pg_attribute instead of going through pg_constraint. This avoids an expensive unnest, window function query, and common table expression. EXPLAINing these queries in Postgres against a database with a single table with a composite primary key shows a 66% reduction in the plan and execute latencies. This is significant during application startup time, especially against very large schemas, where these queries would be even slower and more numerous. Closes #27949
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