[DNM] sql: index pg_catalog.pg_namespace.nspname

[ajwerner]

This adds a virtual index to enable point lookups against pg_namespace.

This approach is even better than just doing a point lookup, it also leverages
leases. That can run into other consistency problems that we aren't going to
think too hard about right now because there's some issues to fix thim in
this release. It's going to turn N lookups and descriptor constructions into
0.

Release note: None

[cockroach-teamcity]

This change is 

[ajwerner]

demo@127.0.0.1:26257/movr> SELECT * FROM pg_catalog.pg_namespace WHERE nspname = 'foo';
     oid     | nspname | nspowner  | nspacl
-------------+---------+-----------+---------
  1521659658 | foo     | 978251175 | NULL
(1 row)


Time: 2ms total (execution 2ms / network 0ms)

demo@127.0.0.1:26257/movr> EXPLAIN SELECT * FROM pg_catalog.pg_namespace WHERE nspname = 'foo';
                       info
--------------------------------------------------
  distribution: local
  vectorized: true

  • virtual table
    table: pg_namespace@pg_namespace_nspname_idx
    spans: [/'foo' - /'foo']
(6 rows)