Re: [HACKERS] time stops within transaction

[Vadim Mikheev]

> > > Snapshot is made per top-level statement and functions/subqueries > > > use the same snapshot as that of top-level statement. > > > > Not so. SetQuerySnapshot is executed per querytree, not per top-level > > statement --- for example, if a rule generates multiple queries from > > a user statement, SetQuerySnapshot is called again for each query. This is true. I just make it to work as it was in pre-6.5 times - each query of *top level* query list uses own snapshot (in read committed mode only) as if they were submitted by user one by one. But functions/subqueries called while executing query uses same snapshot as query itself. > > With the current structure of pg_exec_query_string(), an operation > > executed in the outer loop, rather than the inner, would more or less > > correspond to one "top level" query --- if you want to assume that > > pg_exec_query_string() is only called from PostgresMain. That's > > true today but hasn't always been true --- I believe it used to be > > used to parse SPI commands, and someday it may be again. It was never used in SPI. Just look at _SPI_execute. Same parent query snapshot is used in SPI functions. *But* SPI' queries *see* changes made by parent query - I never was sure about this and think I've asked other opinions. No opinions - no changes -:) > If there's no concept of top-level statement,there's no > concept of read consistency and MVCC isn't needed. Except of the fact that SPI' queries see changes made by parent same snapshot is used all time while executing top-level query (single query, not query list). Vadim