Horton

Re: Minimal logical decoding on standbys

Jeff Davis <pgsql@j-davis.com>

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From: Jeff Davis <pgsql@j-davis.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Amit Khandekar <amitdkhan.pg@gmail.com>, fabriziomello@gmail.com, tushar <tushar.ahuja@enterprisedb.com>, Rahila Syed <rahila.syed@2ndquadrant.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, Melanie Plageman <melanieplageman@gmail.com>
Date: 2023-04-02T20:01:46Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Reduce the log level in 035_standby_logical_decoding.pl.

  2. 035_standby_logical_decoding: Add missing waits for replication

  3. For cascading replication, wake physical and logical walsenders separately

  4. Handle logical slot conflicts on standby

  5. Support invalidating replication slots due to horizon and wal_level

  6. Prevent use of invalidated logical slot in CreateDecodingContext()

  7. Replace replication slot's invalidated_at LSN with an enum

  8. Pass down table relation into more index relation functions

  9. Assert only valid flag bits are passed to visibilitymap_set()

  10. Remove unused _bt_delitems_delete() argument.

  11. Add xl_btree_delete optimization.

On Sun, 2023-04-02 at 10:11 +0200, Drouvot, Bertrand wrote:
> I was thinking that, if a new LogicalWalSndWakeup() replaces
> "ConditionVariableBroadcast(&XLogRecoveryCtl->replayedCV);"
> in ApplyWalRecord() then, it could be possible that some walsender(s)
> are requested to wake up while they are actually doing decoding (but
> I might be wrong).

I don't think that's a problem, right?

We are concerned about wakeups when they happen repeatedly when there's
no work to do, or when the wakeup doesn't happen when it should (and we
need to wait for a timeout).

> > 
> > Currently, we wake up walsenders only after writing some WAL
> > records
> > at the time of flush, so won't it be better to wake up only after
> > applying some WAL records rather than after applying each record?
> 
> Yeah that would be better.

Why? If the walsender is asleep, and there's work to be done, why not
wake it up?

If it's already doing work, and the latch gets repeatedly set, that
doesn't look like a problem either. The comment on SetLatch() says:

  /*
   * Sets a latch and wakes up anyone waiting on it.
   *
   * This is cheap if the latch is already set, otherwise not so much.

Regards,
	Jeff Davis