Re: [HACKERS] time stops within transaction

[Hiroshi Inoue]

Tom Lane wrote: > Hiroshi Inoue <[EMAIL PROTECTED]> writes: > >>>> Postgres doesn't have an idea of what a 'top-level' statement is? I.E. > >>>> statement as submitted by a client (libpq)? > >> > >> There's never been any reason to make such a distinction. > > > There's already a distinction. > > 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. > > 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. > If there's no concept of top-level statement,there's no concept of read consistency and MVCC isn't needed. Regards. Hiroshi Inoue.