sql: add the AOST timestamp to the output of EXPLAIN

[j82w]

The AOST can make a big impact on the performance of a query. It should be included in the explain to avoid confusion.

Jira issue: CRDB-26164
Epic CRDB-37757

[knz]

cc @yuzefovich for triage

[mgartner]

Scans could show that they perform a follow-read.

[knz]

@mgartner where?

demo@127.0.0.1:26257/movr> explain select * from users as of system time '-10s';
            info
-----------------------------
  distribution: local
  vectorized: true

  • scan
    missing stats
    table: users@users_pkey
    spans: FULL SCAN
(7 rows)

I don't see the follow-read there.

[mgartner]

Sorry, I didn't mean that we already show this information, just that we could add it. I think this is more valuable than showing whether or not AOST was used in the query - that's evident in the query itself - and the performance impact is mostly dependent on whether or not a follower read was used, not necessarily whether or not AOST was used, IIUC.