On 1/12/08, alphax <alphax@xxxxxxxxxxx> wrote: > Thanks. Actually, I want to compares the system columns(xmin, xmax, > ctid) with the tid returned by txid functions declared in > > http://developer.postgresql.org/pgdocs/postgres/functions-info.html#FUNCTIONS-TXID-SNAPSHOT > > > I want to determines a given record which visible to current transaction > whether or not be updated after some time point, that time point is > indicated by aother transaction id started and committed in past time. > How can I safely do that? You cannot compare txids with each other and determine visibility, you need snapshots for that. Eg. PgQ (generic queue) is implemented in following way: - Current txid is stored with data rows. - Periodically txid_snapshot is stored to separate table (pgq.tick). - later when reading data, 2 snapshots are taken from pgq.tick, and from them are txids determined that were committed between those. - The data rows for the txids are fetched from data tables then. AFAIK this is only way how to implement robust and high-performance queue in otherwise generic RDBMS. > By the way, Can I think that the value of system column "ctid" of an > record is the logical "current version" of that record, and used to > compare with txid returned by "FUNCTIONS-TXID-SNAPSHOT"? No, it is just physical location of the row. -- marko ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@xxxxxxxxxxxxxx so that your message can get through to the mailing list cleanly
