Обсуждение: now() and statement_timestamp()
According to the documentation. now() should yield the timestamp of the current transaction, while statement_timestamp() should yield the timestamp of the current statement.
In case of Autocommit, both should be identical, in case of a running transaction, they should be different.
Funny enough, psql and executing the same statement from within an application yield different results. I tried a Tcl script and DBeaver.
Statement: SELECT now(), statement_timestamp();
psql: 2021-05-07 13:31:10.135851+02 | 2021-05-07 13:31:10.135851+02
DBeaver: 2021-05-07 13:31:00.633784+02|2021-05-07 13:31:00.633913+02
Tcl script: {now {2021-05-07 13:24:54.659837+02} statement_timestamp {2021-05-07 13:24:54.65985+02}}
So in psql now() and statement_timestamp() are identical when not in a transaction, while in the others the two values vary a little bit, roughly 100 microseconds.
How come the same statements aren't behaving identical. I'm using a current PostgreSQL server on a Linux machine.
-- Holger Jakobs, Bergisch Gladbach, Tel. +49-178-9759012
Вложения
Holger Jakobs <holger@jakobs.com> writes: > So in psql now() and statement_timestamp() are identical when not in a > transaction, while in the others the two values vary a little bit, > roughly 100 microseconds. > How come the same statements aren't behaving identical. I'm using a > current PostgreSQL server on a Linux machine. I'd try turning on log_statement = all to see what's actually happening. What seems somewhat likely is that the non-psql cases are wrapping the command you gave in BEGIN/COMMIT, or something along that line. [ thinks for a bit ] It could also be down to the wire protocol used. psql is just sending a simple Query message, but the other two might well be sending Parse/Bind/Execute. IIRC the transaction must be started by Bind, but we may consider the statement timestamp to be the start of processing Execute. regards, tom lane
Am 27.05.21 um 18:43 schrieb Tom Lane: > Holger Jakobs <holger@jakobs.com> writes: >> So in psql now() and statement_timestamp() are identical when not in a >> transaction, while in the others the two values vary a little bit, >> roughly 100 microseconds. >> How come the same statements aren't behaving identical. I'm using a >> current PostgreSQL server on a Linux machine. > I'd try turning on log_statement = all to see what's actually > happening. What seems somewhat likely is that the non-psql > cases are wrapping the command you gave in BEGIN/COMMIT, > or something along that line. > > [ thinks for a bit ] It could also be down to the wire protocol > used. psql is just sending a simple Query message, but the > other two might well be sending Parse/Bind/Execute. IIRC the > transaction must be started by Bind, but we may consider > the statement timestamp to be the start of processing Execute. > > regards, tom lane I turned on logging of statements, and found out that psql uses a simple query, while the others us an extended query. According to the documentation a simple query and a series of parse, bind, execute are approximately equivalent (Note right above "Function Call" in https://www.postgresql.org/docs/13/protocol-flow.html#id-1.10.5.7.6) So actually the "approximately equivalent" is the point here. The series of parse, bind, execute doesn't call the two funtions now() and statement_timestamp() at the same stage, so the timestamps vary. I wasn't aware of this effect. Now it's clear that one can only use a simple query in order to find out whether a session is within a transaction or in autocommit mode. Maybe the Note in the documentation should mention this clearly. -- Holger Jakobs, Bergisch Gladbach, Tel. +49-178-9759012