sql: Information Schemas

[jseldess]

RFC: cockroachdb/cockroach#7965

PRs:

• sql: Introduce VirtualSchema infrastructure cockroach#8119
• sql: Add initial support for the pg_catalog database cockroach#9565
• sql: Add information_schema.schemata table cockroach#8433
• sql: Add information_schema.table_constraints table cockroach#8498
• add table key_column_usage for information_schema cockroach#9114
• Add schema_privileges and table_privileges two virtual schemas cockroach#9104

[jseldess]

Note from @nvanbenschoten, from #550 (comment):

We could add a section for supported methods of introspection (to SQL Feature Support), but I foresee a page/section like https://www.postgresql.org/docs/9.5/static/information-schema.html being more immediately useful. Each subsection would provide a brief overview of a supported introspection table, along with it's schema.

[jseldess]

@sploiselle, interested in taking on this one? It's actually partly included in the beta-20160829 binary. Some other work may only be on develop.

[jseldess]

@sploiselle, you might want to cover the pg_catalog db as well, which is the postgres-specific version of information_schema. I don't think we have a separate issue for it. Feel free to create one if you'd prefer that.

But based on input from @nvanbenschoten, we should emphasize information_schema since it's part of the SQL standard:

We should be encouraging the use of information_schema wherever possible, because information_schema is part of the SQL spec, while pg_catalog is Postges-specific. The primary main reason we're supporting pg_catalog if for compatibility with Postgres' tools, but I don't think we need to advertise its support.