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@gmail.com> wrote:
>
> On Tue, Apr 28, 2020 at 3:11 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Mon, Apr 27, 2020 at 4:05 PM Dilip Kumar <dilipbalaut@gmail.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)) ?
>
> >
> > 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?

While testing these changes, I have noticed that the systable_* APIs
internally, calls tableam apis and so if we just put assert
Assert(!TransactionIdIsValid(CheckXidAlive)) then it will always hit
that assert.  Whether we put these assert in heap APIs or the tableam
APIs because systable_ always access heap through tableam APIs.

Refer below callstack
#0  table_index_fetch_tuple (scan=0x2392558, tid=0x2392270,
snapshot=0x2392178, slot=0x2391f60, call_again=0x2392276,
all_dead=0x7fff4b6cc89e)
    at ../../../../src/include/access/tableam.h:1035
#1  0x00000000005100b6 in index_fetch_heap (scan=0x2392210,
slot=0x2391f60) at indexam.c:577
#2  0x00000000005101ea in index_getnext_slot (scan=0x2392210,
direction=ForwardScanDirection, slot=0x2391f60) at indexam.c:637
#3  0x000000000050e8f9 in systable_getnext (sysscan=0x2391f08) at genam.c:474
#4  0x0000000000aa44a2 in RelidByRelfilenode (reltablespace=0,
relfilenode=16593) at relfilenodemap.c:213
#5  0x00000000008a64da in ReorderBufferProcessTXN (rb=0x23734b0,
txn=0x2398e28, commit_lsn=23953600, snapshot_now=0x237b168,
command_id=0, streaming=false)
    at reorderbuffer.c:1823
#6  0x00000000008a7201 in ReorderBufferCommit (rb=0x23734b0, xid=518,
commit_lsn=23953600, end_lsn=23953648, commit_time=641466886013448,
origin_id=0, origin_lsn=0)
    at reorderbuffer.c:2315
#7  0x00000000008985b1 in DecodeCommit (ctx=0x22e16a0,
buf=0x7fff4b6cce30, parsed=0x7fff4b6ccca0, xid=518) at decode.c:654
#8  0x0000000000897a76 in DecodeXactOp (ctx=0x22e16a0,
buf=0x7fff4b6cce30) at decode.c:261
#9  0x0000000000897739 in LogicalDecodingProcessRecord (ctx=0x22e16a0,
record=0x22e19a0) at decode.c:130

So basically, the problem is that we can not distinguish whether the
tableam/heap routine is called directly or via systable_*.

Now I understand the current code was actually giving error for the
user table not the system table with the assumption that the system
table will come to this function only via systable_*.  Only user table
can come directly.  So if this is not a system table i.e. we reach
here directly so error out.  Now, I am not sure if it is not for the
system table then what is the purpose of throwing that error?


-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com