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Avoid having two copies of the HOT-chain search logic.

Browse Source 
It's been like this since HOT was originally introduced, but the logic
is complex enough that this is a recipe for bugs, as we've already
found out with SSI.  So refactor heap_hot_search_buffer() so that it
can satisfy the needs of index_getnext(), and make index_getnext() use
that rather than duplicating the logic.

This change was originally proposed by Heikki Linnakangas as part of a
larger refactoring oriented towards allowing index-only scans.  I
extracted and adjusted this part, since it seems to have independent
merit.  Review by Jeff Davis.
This commit is contained in:
Robert Haas
2011-06-27 10:27:17 -04:00
parent 8c8745b298
commit 4da99ea423
6 changed files with 77 additions and 190 deletions
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4
src/backend/access/index/genam.c
View File

@@ -113,9 +113,7 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
ItemPointerSetInvalid(&scan->xs_ctup.t_self);
scan->xs_ctup.t_data = NULL;
scan->xs_cbuf = InvalidBuffer;
scan->xs_hot_dead = false;
scan->xs_next_hot = InvalidOffsetNumber;
scan->xs_prev_xmax = InvalidTransactionId;
scan->xs_continue_hot = false;
return scan;
}

180
src/backend/access/index/indexam.c
View File

@@ -335,7 +335,7 @@ index_rescan(IndexScanDesc scan,
scan->xs_cbuf = InvalidBuffer;
}
scan->xs_next_hot = InvalidOffsetNumber;
scan->xs_continue_hot = false;
scan->kill_prior_tuple = false; /* for safety */
@@ -417,7 +417,7 @@ index_restrpos(IndexScanDesc scan)
SCAN_CHECKS;
GET_SCAN_PROCEDURE(amrestrpos);
scan->xs_next_hot = InvalidOffsetNumber;
scan->xs_continue_hot = false;
scan->kill_prior_tuple = false; /* for safety */
@@ -443,26 +443,18 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
HeapTuple heapTuple = &scan->xs_ctup;
ItemPointer tid = &heapTuple->t_self;
FmgrInfo *procedure;
bool all_dead = false;
SCAN_CHECKS;
GET_SCAN_PROCEDURE(amgettuple);
Assert(TransactionIdIsValid(RecentGlobalXmin));
/*
* We always reset xs_hot_dead; if we are here then either we are just
* starting the scan, or we previously returned a visible tuple, and in
* either case it's inappropriate to kill the prior index entry.
*/
scan->xs_hot_dead = false;
for (;;)
{
OffsetNumber offnum;
bool at_chain_start;
Page dp;
bool got_heap_tuple;
if (scan->xs_next_hot != InvalidOffsetNumber)
if (scan->xs_continue_hot)
{
/*
* We are resuming scan of a HOT chain after having returned an
@@ -471,10 +463,6 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
Assert(BufferIsValid(scan->xs_cbuf));
Assert(ItemPointerGetBlockNumber(tid) ==
BufferGetBlockNumber(scan->xs_cbuf));
Assert(TransactionIdIsValid(scan->xs_prev_xmax));
offnum = scan->xs_next_hot;
at_chain_start = false;
scan->xs_next_hot = InvalidOffsetNumber;
}
else
{
@@ -488,7 +476,7 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
* comments in RelationGetIndexScan().
*/
if (!scan->xactStartedInRecovery)
scan->kill_prior_tuple = scan->xs_hot_dead;
scan->kill_prior_tuple = all_dead;
/*
* The AM's gettuple proc finds the next index entry matching the
@@ -521,151 +509,31 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
if (prev_buf != scan->xs_cbuf)
heap_page_prune_opt(scan->heapRelation, scan->xs_cbuf,
RecentGlobalXmin);
/* Prepare to scan HOT chain starting at index-referenced offnum */
offnum = ItemPointerGetOffsetNumber(tid);
at_chain_start = true;
/* We don't know what the first tuple's xmin should be */
scan->xs_prev_xmax = InvalidTransactionId;
/* Initialize flag to detect if all entries are dead */
scan->xs_hot_dead = true;
}
/* Obtain share-lock on the buffer so we can examine visibility */
LockBuffer(scan->xs_cbuf, BUFFER_LOCK_SHARE);
dp = (Page) BufferGetPage(scan->xs_cbuf);
/* Scan through possible multiple members of HOT-chain */
for (;;)
{
ItemId lp;
ItemPointer ctid;
bool valid;
/* check for bogus TID */
if (offnum < FirstOffsetNumber ||
offnum > PageGetMaxOffsetNumber(dp))
break;
lp = PageGetItemId(dp, offnum);
/* check for unused, dead, or redirected items */
if (!ItemIdIsNormal(lp))
{
/* We should only see a redirect at start of chain */
if (ItemIdIsRedirected(lp) && at_chain_start)
{
/* Follow the redirect */
offnum = ItemIdGetRedirect(lp);
at_chain_start = false;
continue;
}
/* else must be end of chain */
break;
}
/*
* We must initialize all of *heapTuple (ie, scan->xs_ctup) since
* it is returned to the executor on success.
*/
heapTuple->t_data = (HeapTupleHeader) PageGetItem(dp, lp);
heapTuple->t_len = ItemIdGetLength(lp);
ItemPointerSetOffsetNumber(tid, offnum);
heapTuple->t_tableOid = RelationGetRelid(scan->heapRelation);
ctid = &heapTuple->t_data->t_ctid;
/*
* Shouldn't see a HEAP_ONLY tuple at chain start. (This test
* should be unnecessary, since the chain root can't be removed
* while we have pin on the index entry, but let's make it
* anyway.)
*/
if (at_chain_start && HeapTupleIsHeapOnly(heapTuple))
break;
/*
* The xmin should match the previous xmax value, else chain is
* broken. (Note: this test is not optional because it protects
* us against the case where the prior chain member's xmax aborted
* since we looked at it.)
*/
if (TransactionIdIsValid(scan->xs_prev_xmax) &&
!TransactionIdEquals(scan->xs_prev_xmax,
HeapTupleHeaderGetXmin(heapTuple->t_data)))
break;
/* If it's visible per the snapshot, we must return it */
valid = HeapTupleSatisfiesVisibility(heapTuple, scan->xs_snapshot,
scan->xs_cbuf);
CheckForSerializableConflictOut(valid, scan->heapRelation,
heapTuple, scan->xs_cbuf,
scan->xs_snapshot);
if (valid)
{
/*
* If the snapshot is MVCC, we know that it could accept at
* most one member of the HOT chain, so we can skip examining
* any more members. Otherwise, check for continuation of the
* HOT-chain, and set state for next time.
*/
if (IsMVCCSnapshot(scan->xs_snapshot))
scan->xs_next_hot = InvalidOffsetNumber;
else if (HeapTupleIsHotUpdated(heapTuple))
{
Assert(ItemPointerGetBlockNumber(ctid) ==
ItemPointerGetBlockNumber(tid));
scan->xs_next_hot = ItemPointerGetOffsetNumber(ctid);
scan->xs_prev_xmax = HeapTupleHeaderGetXmax(heapTuple->t_data);
}
else
scan->xs_next_hot = InvalidOffsetNumber;
PredicateLockTuple(scan->heapRelation, heapTuple, scan->xs_snapshot);
LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);
pgstat_count_heap_fetch(scan->indexRelation);
return heapTuple;
}
/*
* If we can't see it, maybe no one else can either. Check to see
* if the tuple is dead to all transactions. If we find that all
* the tuples in the HOT chain are dead, we'll signal the index AM
* to not return that TID on future indexscans.
*/
if (scan->xs_hot_dead &&
HeapTupleSatisfiesVacuum(heapTuple->t_data, RecentGlobalXmin,
scan->xs_cbuf) != HEAPTUPLE_DEAD)
scan->xs_hot_dead = false;
/*
* Check to see if HOT chain continues past this tuple; if so
* fetch the next offnum (we don't bother storing it into
* xs_next_hot, but must store xs_prev_xmax), and loop around.
*/
if (HeapTupleIsHotUpdated(heapTuple))
{
Assert(ItemPointerGetBlockNumber(ctid) ==
ItemPointerGetBlockNumber(tid));
offnum = ItemPointerGetOffsetNumber(ctid);
at_chain_start = false;
scan->xs_prev_xmax = HeapTupleHeaderGetXmax(heapTuple->t_data);
}
else
break; /* end of chain */
} /* loop over a single HOT chain */
got_heap_tuple = heap_hot_search_buffer(tid, scan->heapRelation,
scan->xs_cbuf,
scan->xs_snapshot,
&scan->xs_ctup,
&all_dead,
!scan->xs_continue_hot);
LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);
if (got_heap_tuple)
{
/*
* Only in a non-MVCC snapshot can more than one member of the
* HOT chain be visible.
*/
scan->xs_continue_hot = !IsMVCCSnapshot(scan->xs_snapshot);
pgstat_count_heap_fetch(scan->indexRelation);
return heapTuple;
}
/* Loop around to ask index AM for another TID */
scan->xs_next_hot = InvalidOffsetNumber;
scan->xs_continue_hot = false;
}
/* Release any held pin on a heap page */