Track: Evaluation of impure functions

[xiangjinwu]

Opening an issue to track various special functions we have encountered so far. No proposals yet.

• Common
  • Implement now/current_date/current_time/current_timestamp #2870
  • random (pg)
• Depends on session config:
  • cast timestamp to timestamp with time zone (TimeZone Tracking: Exprs blocked by support of implicit TimeZone of session #5826)
  • cast date to string (DateStyle unrecognized configuration parameter "datestyle" #3253)
  • cast float to string (extra_float_digits Support configuration parameter extra_float_digits #3406)
• Depends on pg_catalog and used by psql \d (pg Tracking: System Table in pg_catalog #2954 (comment)):
  • pg_get_userbyid
  • pg_table_is_visible
• pg_sleep: We already decided to remove it refactor: remove function pg_sleep #2576

(Note that expression evaluation does not include aggregates and table functions.)

PostgreSQL classifies functions into 3 categories: immutable, stable and volatile.

[fuyufjh]

+1. Perhaps we need a context when eval

[xxchan]

To summarize, I think we handle impure functions in these ways:

• common: materialization RFC: materialize the result to handle non-deterministic functions rfcs#24
• session config related: some configs simply ignored. General mechanism is rewrite expr to deterministic in optimizer. e.g., SessionTimezone.
• systable related: generally hacked in frontend.

So we can close the issue now?