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Message-ID: <98e01d06-480c-4d27-b880-84480d1bda78@aklaver.com>
Date: Tue, 21 Jan 2025 09:21:12 -0800
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Subject: Re:
To: Lana ABADIE <clipperDB@gmx.fr>, pgsql-general@lists.postgresql.org
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From: Adrian Klaver <adrian.klaver@aklaver.com>
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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.klaver@aklaver.com