[receiver/postgresql] collect query samples only after the newest query sample

[cuichenli]

Description

the current implementation scrape query samples from all the time, which is not very efficiency. this pr changes it to only scrape the query that is newer than the previous oldest query.

Link to tracking issue

Fixes

Testing

Documentation

[atoulme]

@antonblock please review as codeowner

[antonblock]

@cuichenli Could you please merge/rebase to resolve the conflicts?

[cuichenli]

@cuichenli Could you please merge/rebase to resolve the conflicts?

updated

[sincejune]

Actually, I'm wondering if this is the correct thing to do. The query sample collection should collect the current activities, right? even if it has been running for a long time. In this assumption, I think probably duplicated events are expected?

[cuichenli]

The query sample collection should collect the current activities, right? even if it has been running for a long time. In this assumption, I think probably duplicated events are expected?

Yes that is correct, but not what this PR tries to achieve. It is to address that in certain cases we have some idle sessions, those should not be included in the query sample.

query_id bigint
Identifier of this backend's most recent query. If state is active this field shows the identifier of the currently executing query. In all other states, it shows the identifier of last query that was executed.

https://www.postgresql.org/docs/current/monitoring-stats.html#MONITORING-PG-STAT-ACTIVITY-VIEW