Horton

Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Michael Paquier <michael@paquier.xyz>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-04-07T06:47:44Z
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. Tighten the concurrent abort check during decoding.

  2. Improve hash_create()'s API for some added robustness.

  3. Use HASH_BLOBS for xidhash.

  4. Fix initialization of RelationSyncEntry for streaming transactions.

  5. Remove unused function declaration in logicalproto.h.

  6. Add additional tests to test streaming of in-progress transactions.

  7. Fix inline marking introduced in commit 464824323e.

  8. Add support for streaming to built-in logical replication.

  9. Fix the SharedFileSetUnregister API.

  10. Fix comment in procarray.c

  11. Suppress compiler warning in non-cassert builds.

  12. Extend the BufFile interface.

  13. Mark a few logical decoding related variables with PGDLLIMPORT.

  14. Implement streaming mode in ReorderBuffer.

  15. Extend the logical decoding output plugin API with stream methods.

  16. WAL Log invalidations at command end with wal_level=logical.

  17. Immediately WAL-log subtransaction and top-level XID association.

  18. Allow logical replication to transfer data in binary format.

  19. Only superuser can set sslcert/sslkey in postgres_fdw user mappings

  20. Track statistics for spilling of changes from ReorderBuffer.

  21. Add logical_decoding_work_mem to limit ReorderBuffer memory usage.

  22. logical decoding: process ASSIGNMENT during snapshot build

  23. Emit invalidations to standby for transactions without xid.

On Mon, Mar 30, 2020 at 8:58 PM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
>
> On Mon, Mar 30, 2020 at 11:47:57AM +0530, Amit Kapila wrote:
> >
> >I think we need to cache the subxids on the replica somehow but I
> >don't have a very good idea for it.  Basically, there are two ways to
> >do it (a) Change the KnownAssignedXids in some way so that we can
> >easily find this information without losing on the current benefits of
> >it.  I can't think of a good way to do that and even if we come up
> >with something, it could easily be a lot of work, (b) Cache the
> >subxids for a particular transaction in local memory along with
> >KnownAssignedXids.  This is doable but now we have two data-structures
> >(one in shared memory and other in local memory) managing the same
> >information in different ways.
> >
> >Do you have any other ideas?
>
> I don't follow. Why couldn't we have a simple cache on the standby? It
> could be either a simple array or a hash table (with the top-level xid
> as hash key)?
>

I think having something like we discussed or what you have in the
patch won't be sufficient to clean the KnownAssignedXid array. The
point is that we won't write a WAL for xid-subxid association for
unlogged relations in the "Immediately WAL-log assignments" patch,
however, the KnownAssignedXid would have both kinds of Xids as we
autofill it with gaps (see RecordKnownAssignedTransactionIds).  I
think if my understanding is correct to make it work we might need
major surgery in the code or have to maintain KnownAssignedXid array
differently.

>
> I don't think this is particularly complicated or a lot of code, and I
> don't see why would it require data structures in shared memory. Only
> the walreceiver on standby needs to worry about this, no?
>

Not a new data structure in shared memory, but we already have a
KnownTransactionId structure in shared memory. So, after having a
local cache, we will have xidAssignmentsHash and KnownTransactionId
maintaining the same information in different ways.  And, we need to
ensure both are cleaned up properly. That was what I was pointing
above related to maintaining two structures.  However, I think before
discussing more on this, we need to think about the above problem.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com