Horton

Re: Minimal logical decoding on standbys

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>, Jeff Davis <pgsql@j-davis.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: 2023-04-03T06:05:38Z
Lists: pgsql-hackers

Commits

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  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.

Hi,

On 4/3/23 7:20 AM, Amit Kapila wrote:
> On Mon, Apr 3, 2023 at 1:31 AM Jeff Davis <pgsql@j-davis.com> wrote:
>>
>> 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?
>>
> 
> Agreed, I also don't see a problem because of the reason you mentioned
> below that if the latch is already set, we won't do anything in
> SetLatch.

Thanks for the feedback, I do agree too after Jeff's and your explanation.

> 
>> 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?
>>
> 
> I think we can wake it up when there is work to be done even if the
> work unit is smaller. The reason why I mentioned waking up the
> walsender only after processing some records is to avoid the situation
> where it may not need to wait again after decoding very few records.
> But probably the logic in WalSndWaitForWal() will help us to exit
> before starting to wait by checking the replay location.
> 

Okay, I'll re-write the sub-patch related to the startup/walsender corner
case with this new approach.

Regards,


-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com