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

On Wed, May 13, 2020 at 9:16 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Wed, May 13, 2020 at 4:50 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> >
> > > > 4.
> > > > +static void
> > > > +stream_start_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn)
> > > > +{
> > > > + LogicalDecodingContext *ctx = cache->private_data;
> > > > + LogicalErrorCallbackState state;
> > > > + ErrorContextCallback errcallback;
> > > > +
> > > > + Assert(!ctx->fast_forward);
> > > > +
> > > > + /* We're only supposed to call this when streaming is supported. */
> > > > + Assert(ctx->streaming);
> > > > +
> > > > + /* Push callback + info on the error context stack */
> > > > + state.ctx = ctx;
> > > > + state.callback_name = "stream_start";
> > > > + /* state.report_location = apply_lsn; */
> > > >
> > > > Why can't we supply the report_location here?  I think here we need to
> > > > report txn->first_lsn if this is the very first stream and
> > > > txn->final_lsn if it is any consecutive one.
> > >
> > > I am not sure about this,  Because for the very first stream we will
> > > report the location of the first lsn of the stream and for the
> > > consecutive stream we will report the last lsn in the stream.
> > >
> >
> > Yeah, that doesn't seem to be consistent.  How about if get it as an
> > additional parameter?  The caller can pass the lsn of the very first
> > change it is trying to decode in this stream.
>
> Hmm,  I think we need to call ReorderBufferIterTXNInit and
> ReorderBufferIterTXNNext and get the first change of the stream after
> that we shall call stream start then we can find out the first LSN of
> the stream.   I will see how to do so that it doesn't look awkward.
> Basically, as of now, our code is of this layout.
>
> 1. stream_start;
> 2. ReorderBufferIterTXNInit(rb, txn, &iterstate);
> while ((change = ReorderBufferIterTXNNext(rb, iterstate)) != NULL)
> {
> stream changes
> }
> 3. stream stop
>
> So if we want to know the first lsn of this stream then we shall do
> something like this
>
> 1. ReorderBufferIterTXNInit(rb, txn, &iterstate);
> while ((change = ReorderBufferIterTXNNext(rb, iterstate)) != NULL)
> {
>     2. if first_change
>        stream_start;
>
>    stream changes
> }
> 3. stream stop
>

Yeah, something like that would work.  I think you need to see it is
first change for 'streaming' mode.

> > > >
> > > > 11.
> > > > - * HeapTupleSatisfiesHistoricMVCC.
> > > > + * tqual.c's HeapTupleSatisfiesHistoricMVCC.
> > > > + *
> > > > + * We do build the hash table even if there are no CIDs. That's
> > > > + * because when streaming in-progress transactions we may run into
> > > > + * tuples with the CID before actually decoding them. Think e.g. about
> > > > + * INSERT followed by TRUNCATE, where the TRUNCATE may not be decoded
> > > > + * yet when applying the INSERT. So we build a hash table so that
> > > > + * ResolveCminCmaxDuringDecoding does not segfault in this case.
> > > > + *
> > > > + * XXX We might limit this behavior to streaming mode, and just bail
> > > > + * out when decoding transaction at commit time (at which point it's
> > > > + * guaranteed to see all CIDs).
> > > >   */
> > > >  static void
> > > >  ReorderBufferBuildTupleCidHash(ReorderBuffer *rb, ReorderBufferTXN *txn)
> > > > @@ -1350,9 +1498,6 @@ ReorderBufferBuildTupleCidHash(ReorderBuffer
> > > > *rb, ReorderBufferTXN *txn)
> > > >   dlist_iter iter;
> > > >   HASHCTL hash_ctl;
> > > >
> > > > - if (!rbtxn_has_catalog_changes(txn) || dlist_is_empty(&txn->tuplecids))
> > > > - return;
> > > > -
> > > >
> > > > I don't understand this change.  Why would "INSERT followed by
> > > > TRUNCATE" could lead to a tuple which can come for decode before its
> > > > CID?
> > >
> > > Actually, even if we haven't decoded the DDL operation but in the
> > > actual system table the tuple might have been deleted from the next
> > > operation.  e.g. while we are streaming the INSERT it is possible that
> > > the truncate has already deleted that tuple and set the max for the
> > > tuple.  So before streaming patch, we were only streaming the INSERT
> > > only on commit so by that time we had got all the operation which has
> > > done DDL and we would have already prepared tuple CID hash.
> > >
> >
> > Okay, but I think for that case how good is that we always allow CID
> > hash table to be built even if there are no catalog changes in TXN
> > (see changes in ReorderBufferBuildTupleCidHash).  Can't we detect that
> > while resolving the cmin/cmax?
>
> Maybe in ResolveCminCmaxDuringDecoding we can see if tuplecid_data is
> NULL then we can return as unresolved and then caller can take a call
> based on that.
>

Yeah, and add appropriate comments about why we are doing so and in
what kind of scenario that can happen.

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