Commits on Jul 5, 2023

1. adapter: correct initial timestamp determination in transactions …

   In MaterializeInc#19026 various timestamp functions were refactored to allow `EXPLAIN
   TIMESTAMP` to reflect (and effect) the current transaction. This is
   tricky code and the refactor incorrectly changed the behavior of
   transactions. In a transaction our intention is to find a timestamp
   during the first statement that will be valid for any other object a
   user might query later in the transaction, then aquire read holds on all
   those referenced objects. In that change, the first statement only used
   the referenced IDs when determining a timestamp, not all nearby (objects
   in the same schema) IDs. Read holds were still correctly aquired on all
   nearby objects.
   
   This could cause the transaction's timestamp to not be in advance of the
   since of other nearby objects. For example, take two objects in the
   table. At the first query, object A has a since of 10, object B has a
   since of 20. A first query in the transaction `SELECT * FROM A` would
   choose 10 as its timestamp, then aquire read holds on A and B at 10. A
   later query in the transaction `SELECT * FROM B` would choose 10 as its
   timestamp (which it fetched from the in-progress transaction state), and
   panic because B's since is in advance of 10.
   
   One of the reasons this happened is the transaction metadata only held
   on to the TimestampContext. `EXPLAIN TIMESTAMP` needs a larger struct, a
   TimestampDetermination. Teach the session metadata to track this outer
   struct so EXPLAIN can use it. This allows us to now only call
   determine_timestamp once during transaction start, instead of during
   each query in a transaction. Refactor some of the other read handling
   code to look more like it did pre-19026.

   

   maddyblue committed Jul 5, 2023
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