Re: 9.2.1 & index-only scans : abnormal heap fetches after VACUUM FULL

On Sat, Feb 15, 2014 at 04:16:40PM +0100, Andres Freund wrote:
> Hi,
>
> > *************** end_heap_rewrite(RewriteState state)
> > *** 281,286 ****
> > --- 284,290 ----
> > true);
> > RelationOpenSmgr(state->rs_new_rel);
> >
> > + update_page_vm(state->rs_new_rel, state->rs_buffer, state->rs_blockno);
> > PageSetChecksumInplace(state->rs_buffer, state->rs_blockno);
> >
> > smgrextend(state->rs_new_rel->rd_smgr, MAIN_FORKNUM, state->rs_blockno,
> > *************** raw_heap_insert(RewriteState state, Heap
> > *** 633,638 ****
> > --- 637,643 ----
> > */
> > RelationOpenSmgr(state->rs_new_rel);
> >
> > + update_page_vm(state->rs_new_rel, page, state->rs_blockno);
> > PageSetChecksumInplace(page, state->rs_blockno);
> >
> > smgrextend(state->rs_new_rel->rd_smgr, MAIN_FORKNUM,
>
> I think those two cases should be combined.

Uh, what I did was to pair the new update_page_vm with the existing
PageSetChecksumInplace calls, figuring if we needed a checksum before we
wrote the page we would need a update_page_vm too. Is that incorrect?
It is that _last_ page write in the second instance.

> > + static void
> > + update_page_vm(Relation relation, Page page, BlockNumber blkno)
> > + {
> > + Buffer vmbuffer = InvalidBuffer;
> > + TransactionId visibility_cutoff_xid;
> > +
> > + visibilitymap_pin(relation, blkno, &vmbuffer);
> > + Assert(BufferIsValid(vmbuffer));
> > +
> > + if (!visibilitymap_test(relation, blkno, &vmbuffer) &&
> > + heap_page_is_all_visible(relation, InvalidBuffer, page,
> > + &visibility_cutoff_xid))
> > + {
> > + PageSetAllVisible(page);
> > + visibilitymap_set(relation, blkno, InvalidBuffer,
> > + InvalidXLogRecPtr, vmbuffer, visibility_cutoff_xid);
> > + }
> > + ReleaseBuffer(vmbuffer);
> > + }
>
> How could visibilitymap_test() be true here?

Oh, you are right that I can only hit that once per page; test removed.

> > diff --git a/src/backend/access/heap/visibilitymap.c b/src/backend/access/heap/visibilitymap.c
> > new file mode 100644
> > index 899ffac..3e7546b
> > *** a/src/backend/access/heap/visibilitymap.c
> > --- b/src/backend/access/heap/visibilitymap.c
> > *************** visibilitymap_set(Relation rel, BlockNum
> > *** 257,263 ****
> > #endif
> >
> > Assert(InRecovery || XLogRecPtrIsInvalid(recptr));
> > - Assert(InRecovery || BufferIsValid(heapBuf));
> >
> > /* Check that we have the right heap page pinned, if present */
> > if (BufferIsValid(heapBuf) && BufferGetBlockNumber(heapBuf) != heapBlk)
> > --- 257,262 ----
> > *************** visibilitymap_set(Relation rel, BlockNum
> > *** 278,284 ****
> > map[mapByte] |= (1 << mapBit);
> > MarkBufferDirty(vmBuf);
> >
> > ! if (RelationNeedsWAL(rel))
> > {
> > if (XLogRecPtrIsInvalid(recptr))
> > {
> > --- 277,283 ----
> > map[mapByte] |= (1 << mapBit);
> > MarkBufferDirty(vmBuf);
> >
> > ! if (RelationNeedsWAL(rel) && BufferIsValid(heapBuf))
> > {
> > if (XLogRecPtrIsInvalid(recptr))
> > {
>
> At the very least this needs to revise visibilitymap_set's comment about
> the requirement of passing a buffer unless !InRecovery.

Oh, good point, comment block updated.

> Also, how is this going to work with replication if you're explicitly
> not WAL logging this?

Uh, I assumed that using the existing functions would handle this. If
not, I don't know the answer.

> > *************** vac_cmp_itemptr(const void *left, const
> > *** 1730,1743 ****
> > * transactions. Also return the visibility_cutoff_xid which is the highest
> > * xmin amongst the visible tuples.
> > */
> > ! static bool
> > ! heap_page_is_all_visible(Relation rel, Buffer buf, TransactionId *visibility_cutoff_xid)
> > {
> > - Page page = BufferGetPage(buf);
> > OffsetNumber offnum,
> > maxoff;
> > bool all_visible = true;
> >
> > *visibility_cutoff_xid = InvalidTransactionId;
> >
> > /*
> > --- 1728,1744 ----
> > * transactions. Also return the visibility_cutoff_xid which is the highest
> > * xmin amongst the visible tuples.
> > */
> > ! bool
> > ! heap_page_is_all_visible(Relation rel, Buffer buf, Page page,
> > ! TransactionId *visibility_cutoff_xid)
> > {
> > OffsetNumber offnum,
> > maxoff;
> > bool all_visible = true;
> >
> > + if (BufferIsValid(buf))
> > + page = BufferGetPage(buf);
> > +
> > *visibility_cutoff_xid = InvalidTransactionId;
> >
> > /*
> > *************** heap_page_is_all_visible(Relation rel, B
> > *** 1758,1764 ****
> > if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid))
> > continue;
> >
> > ! ItemPointerSet(&(tuple.t_self), BufferGetBlockNumber(buf), offnum);
> >
> > /*
> > * Dead line pointers can have index pointers pointing to them. So
> > --- 1759,1767 ----
> > if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid))
> > continue;
> >
> > ! /* XXX use 0 or real offset? */
> > ! ItemPointerSet(&(tuple.t_self), BufferIsValid(buf) ?
> > ! BufferGetBlockNumber(buf) : 0, offnum);
>
> How about passing in the page and block number from the outside and not
> dealing with a buffer in here at all?

I would love to do that but HeapTupleSatisfiesVacuum() requires a
buffer, though you can (with my patch) optionally not supply one,
meaning if I passed in just the block number I would still need to
generate a buffer pointer.

> > /*
> > * Dead line pointers can have index pointers pointing to them. So
> > diff --git a/src/backend/utils/time/tqual.c b/src/backend/utils/time/tqual.c
> > new file mode 100644
> > index f626755..b37ecc4
> > *** a/src/backend/utils/time/tqual.c
> > --- b/src/backend/utils/time/tqual.c
> > *************** static inline void
> > *** 108,113 ****
> > --- 108,117 ----
> > SetHintBits(HeapTupleHeader tuple, Buffer buffer,
> > uint16 infomask, TransactionId xid)
> > {
> > + /* we might not have a buffer if we are doing raw_heap_insert() */
> > + if (!BufferIsValid(buffer))
> > + return;
> > +
> > if (TransactionIdIsValid(xid))
> > {
> > /* NB: xid must be known committed here! */
>
> This looks seriously wrong to me.
>
> I think the whole idea of doing this in private memory is bad. The
> visibility routines aren't written for that, and the above is just one
> instance of that, and I am far from convinced it's the only one. So you
> either need to work out how to rely on the visibility checking done in
> cluster.c, or you need to modify rewriteheap.c to write via
> shared_buffers.

I don't think we can change rewriteheap.c to use shared buffers --- that
code was written to do things in private memory for performance reasons
and I don't think setting hit bits justifies changing that.

Can you be more specific about the cluster.c idea? I see
copy_heap_data() in cluster.c calling HeapTupleSatisfiesVacuum() with a
'buf' just like the code I am working in.

Based on Robert's feedback a few months ago I suspected I would need
help completing this patch --- now I am sure.

Updated patch attached.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +