Add partitioned table model

What does this MR do?

Related to #241267 (closed)

Various pieces of the code that deal with partitioning have to query the state of the partitioned tables and their partitions. Over time, this runs the risk of repeating the same (or very similar) queries over and over. Not only is this repetitive, but the postgres system catalogs can be difficult to decipher, so it's best to abstract them with something more easily understood.

This MR follows a pattern first discussed and introduced here: !42967 (merged), and introduces an AR model backed by a database view that provides an easy way to query the Postgres system catalogs to find information about partitioned tables.

A second MR !45592 (merged) expands on this work to add a view/model that can be used to query the table's partitions.

Migration up

rails db:migrate:up VERSION=20201019161924
== 20201019161924 AddPartitionedTableView: migrating ==========================
-- execute("CREATE OR REPLACE VIEW postgres_partitioned_tables AS\nSELECT\n  pg_namespace.nspname::text || '.'::text || pg_class.relname::text AS identifier,\n  pg_class.oid AS oid,\n  pg_namespace.nspname AS schema,\n  pg_class.relname AS name,\n  CASE partitioned_tables.partstrat\n  WHEN 'l' THEN 'list'\n  WHEN 'r' THEN 'range'\n  WHEN 'h' THEN 'hash'\n  END as strategy,\n  array_agg(pg_attribute.attname) as key_columns\n  FROM (\n    SELECT\n      partrelid,\n      partstrat,\n      unnest(partattrs) as column_position\n    FROM pg_partitioned_table\n  ) partitioned_tables\n  INNER JOIN pg_class\n  ON partitioned_tables.partrelid = pg_class.oid\n  INNER JOIN pg_namespace\n  ON pg_class.relnamespace = pg_namespace.oid\n  INNER JOIN pg_attribute\n  ON pg_attribute.attrelid = pg_class.oid\n  AND pg_attribute.attnum = partitioned_tables.column_position\n  WHERE pg_namespace.nspname = current_schema()\n  GROUP BY identifier, pg_class.oid, schema, name, strategy;\n")
   -> 0.0026s
== 20201019161924 AddPartitionedTableView: migrated (0.0027s) =================

Migration down

rails db:migrate:down VERSION=20201019161924
== 20201019161924 AddPartitionedTableView: reverting ==========================
-- execute("DROP VIEW IF EXISTS postgres_partitioned_tables\n")
   -> 0.0016s
== 20201019161924 AddPartitionedTableView: reverted (0.0017s) =================

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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