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Re: Minimal logical decoding on standbys

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>
Cc: 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>
Date: 2022-12-15T10:20:18Z
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.

Hi,

On 12/14/22 6:48 PM, Robert Haas wrote:
> On Wed, Dec 14, 2022 at 12:35 PM Andres Freund <andres@anarazel.de> wrote:
>>>   typedef struct xl_heap_prune
>>>
>>> I think this is unsafe on alignment-picky machines. I think it will
>>> cause the offset numbers to be aligned at an odd address.
>>> heap_xlog_prune() doesn't copy the data into aligned memory, so I
>>> think this will result in a misaligned pointer being passed down to
>>> heap_page_prune_execute.
>>
>> I think the offset numbers are stored separately from the record, even
>> though it doesn't quite look like that in the above due to the way the
>> 'OFFSET NUMBERS' is embedded in the struct. As they're stored with the
>> block reference 0, the added boolean shouldn't make a difference
>> alignment wise?
>>
>> Or am I misunderstanding your point?
> 
> Oh, you're right. So this is another case similar to
> xl_btree_reuse_page. In heap_xlog_prune(), we access the offset number
> data like this:
> 
>                  redirected = (OffsetNumber *)
> XLogRecGetBlockData(record, 0, &datalen);
>                  end = (OffsetNumber *) ((char *) redirected + datalen);
>                  nowdead = redirected + (nredirected * 2);
>                  nowunused = nowdead + ndead;
>                  nunused = (end - nowunused);
>                  heap_page_prune_execute(buffer,
> 
> redirected, nredirected,
>                                                                  nowdead, ndead,
> 
> nowunused, nunused);
> 
> This is only safe if the return value of XLogRecGetBlockData is
> guaranteed to be properly aligned,

Why, could you please elaborate?

It looks to me that here we are "just" accessing the
members of the xl_heap_prune struct to get the numbers.

Then, the actual data will be read later in heap_page_prune_execute() from the buffer/page based on the numbers we got from xl_heap_prune.

Regards,

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