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

On Tue, Apr 28, 2020 at 3:55 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Tue, Apr 28, 2020 at 3:11 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Mon, Apr 27, 2020 at 4:05 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > >
> > [latest patches]
> >
> > v16-0004-Gracefully-handle-concurrent-aborts-of-uncommitt
> > - Any actions leading to transaction ID assignment are prohibited.
> > That, among others,
> > + Note that access to user catalog tables or regular system catalog tables
> > + in the output plugins has to be done via the
> > <literal>systable_*</literal> scan APIs only.
> > + Access via the <literal>heap_*</literal> scan APIs will error out.
> > + Additionally, any actions leading to transaction ID assignment
> > are prohibited. That, among others,
> > ..
> > @@ -1383,6 +1392,14 @@ heap_fetch(Relation relation,
> > bool valid;
> >
> > /*
> > + * We don't expect direct calls to heap_fetch with valid
> > + * CheckXidAlive for regular tables. Track that below.
> > + */
> > + if (unlikely(TransactionIdIsValid(CheckXidAlive) &&
> > + !(IsCatalogRelation(relation) || RelationIsUsedAsCatalogTable(relation))))
> > + elog(ERROR, "unexpected heap_fetch call during logical decoding");
> > +
> >
> > I think comments and code don't match. In the comment, we are saying
> > that via output plugins access to user catalog tables or regular
> > system catalog tables won't be allowed via heap_* APIs but code
> > doesn't seem to reflect it. I feel only
> > TransactionIdIsValid(CheckXidAlive) is sufficient here. See, the
> > original discussion about this point [1] (Refer "I think it'd also be
> > good to add assertions to codepaths not going through systable_*
> > asserting that ...").
>
> Right, So I think we can just add an assert in these function that
> Assert(!TransactionIdIsValid(CheckXidAlive)) ?
>

I am fine with Assertion but update the documentation accordingly.
However, I think you can once cross-verify if there are any output
plugins that are already using such APIs. There is a list of "Logical
Decoding Plugins" on the wiki [1], just look into those once.

> >
> > Isn't it better to block the scan to user catalog tables or regular
> > system catalog tables for tableam scan APIs rather than at the heap
> > level? There might be some APIs like heap_getnext where such a check
> > might still be required but I guess it is still better to block at
> > tableam level.
> >
> > [1] - https://www.postgresql.org/message-id/20180726200241.aje4dv4jsv25v4k2%40alap3.anarazel.de
>
> Okay, let me analyze this part. Because someplace we have to keep at
> heap level like heap_getnext and other places at tableam level so it
> seems a bit inconsistent. Also, I think the number of checks might
> going to increase because some of the heap functions like
> heap_hot_search_buffer are being called from multiple tableam calls,
> so we need to put check at every place.
>
> Another point is that I feel some of the checks what we have today
> might not be required like heap_finish_speculative, is not fetching
> any tuple for us so why do we need to care about this function?
>

Yeah, I don't see the need for such a check (or Assertion) in
heap_finish_speculative.

One additional comment:
---------------------------------------
- Any actions leading to transaction ID assignment are prohibited.
That, among others,
+ Note that access to user catalog tables or regular system catalog tables
+ in the output plugins has to be done via the
<literal>systable_*</literal> scan APIs only.
+ Access via the <literal>heap_*</literal> scan APIs will error out.
+ Additionally, any actions leading to transaction ID assignment
are prohibited. That, among others,

The above text doesn't seem to be aligned properly and you need to
update it if we want to change the error to Assertion for heap APIs

[1] - https://wiki.postgresql.org/wiki/Logical_Decoding_Plugins

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