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help / color / mirror / Atom feedFrom: Lana ABADIE <[email protected]>
To: [email protected]
Date: Mon, 20 Jan 2025 19:10:15 +0100
Message-ID: <trinity-b4157c97-6383-4d11-b32a-e6f48a7b1baa-1737396615352@3c-app-mailcom-bs15> (raw)
<html><head></head><body><div style="font-family: Verdana;font-size: 12.0px;"><div>Hi all </div>
<div>I bumped into a weird case that i don't really understand...maybe someone in this list could have a clue</div>
<div>We have 2 Postgres databases configured as master/slave replica (Postgresq 12, RHEL8)</div>
<div>We have applications which write data into the master and applications which reads data from the replica.</div>
<div>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.</div>
<div>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.</div>
<div>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;</div>
<div>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...</div>
<div>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,</div>
<div>Note that we are talking about a total number of rows less than 2000.</div>
<div>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.</div>
<div>I also confirmed via metrics collection that the data is there when the query is being performed.</div>
<div>I would appreciate any explanations on this behavior, and hoping i'm clear.</div>
<div>Thanks</div>
<div>Doris</div></div></body></html>
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