Re:

On 1/20/25 10:10, Lana ABADIE wrote:
> Hi all
> I bumped into a weird case that i don't really understand...maybe
> someone in this list could have a clue
> We have 2 Postgres databases configured as master/slave
> replica (Postgresq 12, RHEL8)
> We have applications which write data into the master and applications
> which reads data from the replica.
> A group of applications reads data using libpq: it declares a select
> statement as cursor and then there is fetch which can retrieve at most
> 25k rows.

Add the complete SELECT and CURSOR code.

> The select statement contains a between clause with T1 and T2. T1 is
> injected via input parameter but T2=floor(extract (epoch from
> coalesce(pg_last_xact_replay_timestamp(),now())))-120. It is passed
> directly like that in the query.

What does T1 represent and how is it derived?

> In other words we have something like select * from ZZ where ... and
> timestamp between $T1 and floor(extract (epoch from
> coalesce(pg_last_xact_replay_timestamp(),now())))-120;
> when this query gets executed, from time to time it returns a truncated
> number of rows.. less than if i was doing between T1 and T1...

I don't understand above, add more complete definition. Example data
would be nice.

> T2 being an integer so either T2<T1 in that case i would get a number of
> rows of zero or T2>=T1 and I would expect at least #rows greater or
> equal to the number of rows between T1 and T1,
> Note that we are talking about  a total number of rows less than 2000.
> Then when i fixed T2, in other words i do a query using between $T1 and
> $T2 (where T2=floor(extract (epoch from
> coalesce(pg_last_xact_replay_timestamp(),now())))-120) then there is no
> issues, number of rows are retrieved correctly.
> I also confirmed via metrics collection that the data is there when the
> query is being performed.
> I would appreciate any explanations on this behavior, and hoping i'm clear.
> Thanks
> Doris

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com