mirror of https://github.com/postgres/postgres
Avoid having two copies of the HOT-chain search logic.
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
parent
8c8745b298
commit
4da99ea423
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@ -1514,6 +1514,10 @@ heap_fetch(Relation relation,
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* found, we update *tid to reference that tuple's offset number, and
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* return TRUE. If no match, return FALSE without modifying *tid.
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*
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* heapTuple is a caller-supplied buffer. When a match is found, we return
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* the tuple here, in addition to updating *tid. If no match is found, the
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* contents of this buffer on return are undefined.
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*
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* If all_dead is not NULL, we check non-visible tuples to see if they are
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* globally dead; *all_dead is set TRUE if all members of the HOT chain
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* are vacuumable, FALSE if not.
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@ -1524,28 +1528,31 @@ heap_fetch(Relation relation,
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*/
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bool
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heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
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Snapshot snapshot, bool *all_dead)
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Snapshot snapshot, HeapTuple heapTuple,
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bool *all_dead, bool first_call)
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{
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Page dp = (Page) BufferGetPage(buffer);
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TransactionId prev_xmax = InvalidTransactionId;
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OffsetNumber offnum;
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bool at_chain_start;
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bool valid;
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bool skip;
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/* If this is not the first call, previous call returned a (live!) tuple */
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if (all_dead)
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*all_dead = true;
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*all_dead = first_call;
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Assert(TransactionIdIsValid(RecentGlobalXmin));
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Assert(ItemPointerGetBlockNumber(tid) == BufferGetBlockNumber(buffer));
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offnum = ItemPointerGetOffsetNumber(tid);
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at_chain_start = true;
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at_chain_start = first_call;
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skip = !first_call;
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/* Scan through possible multiple members of HOT-chain */
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for (;;)
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{
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ItemId lp;
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HeapTupleData heapTuple;
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/* check for bogus TID */
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if (offnum < FirstOffsetNumber || offnum > PageGetMaxOffsetNumber(dp))
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@ -1568,15 +1575,15 @@ heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
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break;
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}
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heapTuple.t_data = (HeapTupleHeader) PageGetItem(dp, lp);
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heapTuple.t_len = ItemIdGetLength(lp);
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heapTuple.t_tableOid = relation->rd_id;
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heapTuple.t_self = *tid;
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heapTuple->t_data = (HeapTupleHeader) PageGetItem(dp, lp);
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heapTuple->t_len = ItemIdGetLength(lp);
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heapTuple->t_tableOid = relation->rd_id;
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heapTuple->t_self = *tid;
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/*
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* Shouldn't see a HEAP_ONLY tuple at chain start.
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*/
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if (at_chain_start && HeapTupleIsHeapOnly(&heapTuple))
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if (at_chain_start && HeapTupleIsHeapOnly(heapTuple))
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break;
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/*
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@ -1585,21 +1592,32 @@ heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
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*/
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if (TransactionIdIsValid(prev_xmax) &&
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!TransactionIdEquals(prev_xmax,
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HeapTupleHeaderGetXmin(heapTuple.t_data)))
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HeapTupleHeaderGetXmin(heapTuple->t_data)))
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break;
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/*
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* When first_call is true (and thus, skip is initally false) we'll
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* return the first tuple we find. But on later passes, heapTuple
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* will initially be pointing to the tuple we returned last time.
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* Returning it again would be incorrect (and would loop forever),
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* so we skip it and return the next match we find.
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*/
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if (!skip)
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{
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/* If it's visible per the snapshot, we must return it */
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valid = HeapTupleSatisfiesVisibility(&heapTuple, snapshot, buffer);
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CheckForSerializableConflictOut(valid, relation, &heapTuple, buffer,
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snapshot);
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valid = HeapTupleSatisfiesVisibility(heapTuple, snapshot, buffer);
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CheckForSerializableConflictOut(valid, relation, heapTuple,
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buffer, snapshot);
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if (valid)
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{
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ItemPointerSetOffsetNumber(tid, offnum);
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PredicateLockTuple(relation, &heapTuple, snapshot);
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PredicateLockTuple(relation, heapTuple, snapshot);
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if (all_dead)
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*all_dead = false;
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return true;
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}
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}
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skip = false;
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/*
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* If we can't see it, maybe no one else can either. At caller
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@ -1607,7 +1625,7 @@ heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
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* transactions.
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*/
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if (all_dead && *all_dead &&
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HeapTupleSatisfiesVacuum(heapTuple.t_data, RecentGlobalXmin,
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HeapTupleSatisfiesVacuum(heapTuple->t_data, RecentGlobalXmin,
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buffer) != HEAPTUPLE_DEAD)
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*all_dead = false;
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@ -1615,13 +1633,13 @@ heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
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* Check to see if HOT chain continues past this tuple; if so fetch
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* the next offnum and loop around.
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*/
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if (HeapTupleIsHotUpdated(&heapTuple))
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if (HeapTupleIsHotUpdated(heapTuple))
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{
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Assert(ItemPointerGetBlockNumber(&heapTuple.t_data->t_ctid) ==
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Assert(ItemPointerGetBlockNumber(&heapTuple->t_data->t_ctid) ==
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ItemPointerGetBlockNumber(tid));
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offnum = ItemPointerGetOffsetNumber(&heapTuple.t_data->t_ctid);
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offnum = ItemPointerGetOffsetNumber(&heapTuple->t_data->t_ctid);
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at_chain_start = false;
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prev_xmax = HeapTupleHeaderGetXmax(heapTuple.t_data);
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prev_xmax = HeapTupleHeaderGetXmax(heapTuple->t_data);
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}
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else
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break; /* end of chain */
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@ -1643,10 +1661,12 @@ heap_hot_search(ItemPointer tid, Relation relation, Snapshot snapshot,
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{
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bool result;
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Buffer buffer;
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HeapTupleData heapTuple;
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buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));
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LockBuffer(buffer, BUFFER_LOCK_SHARE);
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result = heap_hot_search_buffer(tid, relation, buffer, snapshot, all_dead);
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result = heap_hot_search_buffer(tid, relation, buffer, snapshot,
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&heapTuple, all_dead, true);
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LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
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ReleaseBuffer(buffer);
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return result;
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@ -113,9 +113,7 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
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ItemPointerSetInvalid(&scan->xs_ctup.t_self);
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scan->xs_ctup.t_data = NULL;
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scan->xs_cbuf = InvalidBuffer;
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scan->xs_hot_dead = false;
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scan->xs_next_hot = InvalidOffsetNumber;
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scan->xs_prev_xmax = InvalidTransactionId;
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scan->xs_continue_hot = false;
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return scan;
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}
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@ -335,7 +335,7 @@ index_rescan(IndexScanDesc scan,
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scan->xs_cbuf = InvalidBuffer;
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}
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scan->xs_next_hot = InvalidOffsetNumber;
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scan->xs_continue_hot = false;
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scan->kill_prior_tuple = false; /* for safety */
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@ -417,7 +417,7 @@ index_restrpos(IndexScanDesc scan)
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SCAN_CHECKS;
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GET_SCAN_PROCEDURE(amrestrpos);
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scan->xs_next_hot = InvalidOffsetNumber;
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scan->xs_continue_hot = false;
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scan->kill_prior_tuple = false; /* for safety */
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@ -443,26 +443,18 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
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HeapTuple heapTuple = &scan->xs_ctup;
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ItemPointer tid = &heapTuple->t_self;
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FmgrInfo *procedure;
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bool all_dead = false;
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SCAN_CHECKS;
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GET_SCAN_PROCEDURE(amgettuple);
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Assert(TransactionIdIsValid(RecentGlobalXmin));
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/*
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* We always reset xs_hot_dead; if we are here then either we are just
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* starting the scan, or we previously returned a visible tuple, and in
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* either case it's inappropriate to kill the prior index entry.
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*/
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scan->xs_hot_dead = false;
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for (;;)
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{
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OffsetNumber offnum;
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bool at_chain_start;
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Page dp;
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bool got_heap_tuple;
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if (scan->xs_next_hot != InvalidOffsetNumber)
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if (scan->xs_continue_hot)
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{
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/*
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* We are resuming scan of a HOT chain after having returned an
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@ -471,10 +463,6 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
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Assert(BufferIsValid(scan->xs_cbuf));
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Assert(ItemPointerGetBlockNumber(tid) ==
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BufferGetBlockNumber(scan->xs_cbuf));
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Assert(TransactionIdIsValid(scan->xs_prev_xmax));
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offnum = scan->xs_next_hot;
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at_chain_start = false;
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scan->xs_next_hot = InvalidOffsetNumber;
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}
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else
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{
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@ -488,7 +476,7 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
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* comments in RelationGetIndexScan().
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*/
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if (!scan->xactStartedInRecovery)
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scan->kill_prior_tuple = scan->xs_hot_dead;
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scan->kill_prior_tuple = all_dead;
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/*
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* The AM's gettuple proc finds the next index entry matching the
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@ -521,151 +509,31 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
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if (prev_buf != scan->xs_cbuf)
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heap_page_prune_opt(scan->heapRelation, scan->xs_cbuf,
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RecentGlobalXmin);
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/* Prepare to scan HOT chain starting at index-referenced offnum */
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offnum = ItemPointerGetOffsetNumber(tid);
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at_chain_start = true;
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/* We don't know what the first tuple's xmin should be */
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scan->xs_prev_xmax = InvalidTransactionId;
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/* Initialize flag to detect if all entries are dead */
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scan->xs_hot_dead = true;
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}
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/* Obtain share-lock on the buffer so we can examine visibility */
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LockBuffer(scan->xs_cbuf, BUFFER_LOCK_SHARE);
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dp = (Page) BufferGetPage(scan->xs_cbuf);
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/* Scan through possible multiple members of HOT-chain */
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for (;;)
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{
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ItemId lp;
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ItemPointer ctid;
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bool valid;
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/* check for bogus TID */
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if (offnum < FirstOffsetNumber ||
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offnum > PageGetMaxOffsetNumber(dp))
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break;
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lp = PageGetItemId(dp, offnum);
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/* check for unused, dead, or redirected items */
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if (!ItemIdIsNormal(lp))
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{
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/* We should only see a redirect at start of chain */
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if (ItemIdIsRedirected(lp) && at_chain_start)
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{
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/* Follow the redirect */
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offnum = ItemIdGetRedirect(lp);
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at_chain_start = false;
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continue;
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}
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/* else must be end of chain */
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break;
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}
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/*
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* We must initialize all of *heapTuple (ie, scan->xs_ctup) since
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* it is returned to the executor on success.
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*/
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heapTuple->t_data = (HeapTupleHeader) PageGetItem(dp, lp);
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heapTuple->t_len = ItemIdGetLength(lp);
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ItemPointerSetOffsetNumber(tid, offnum);
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heapTuple->t_tableOid = RelationGetRelid(scan->heapRelation);
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ctid = &heapTuple->t_data->t_ctid;
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/*
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* Shouldn't see a HEAP_ONLY tuple at chain start. (This test
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* should be unnecessary, since the chain root can't be removed
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* while we have pin on the index entry, but let's make it
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* anyway.)
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*/
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if (at_chain_start && HeapTupleIsHeapOnly(heapTuple))
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break;
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/*
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* The xmin should match the previous xmax value, else chain is
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* broken. (Note: this test is not optional because it protects
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* us against the case where the prior chain member's xmax aborted
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* since we looked at it.)
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*/
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if (TransactionIdIsValid(scan->xs_prev_xmax) &&
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!TransactionIdEquals(scan->xs_prev_xmax,
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HeapTupleHeaderGetXmin(heapTuple->t_data)))
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break;
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/* If it's visible per the snapshot, we must return it */
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valid = HeapTupleSatisfiesVisibility(heapTuple, scan->xs_snapshot,
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scan->xs_cbuf);
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CheckForSerializableConflictOut(valid, scan->heapRelation,
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heapTuple, scan->xs_cbuf,
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scan->xs_snapshot);
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if (valid)
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{
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/*
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* If the snapshot is MVCC, we know that it could accept at
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* most one member of the HOT chain, so we can skip examining
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* any more members. Otherwise, check for continuation of the
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* HOT-chain, and set state for next time.
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*/
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if (IsMVCCSnapshot(scan->xs_snapshot))
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scan->xs_next_hot = InvalidOffsetNumber;
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else if (HeapTupleIsHotUpdated(heapTuple))
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{
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Assert(ItemPointerGetBlockNumber(ctid) ==
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ItemPointerGetBlockNumber(tid));
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scan->xs_next_hot = ItemPointerGetOffsetNumber(ctid);
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scan->xs_prev_xmax = HeapTupleHeaderGetXmax(heapTuple->t_data);
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}
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else
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scan->xs_next_hot = InvalidOffsetNumber;
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PredicateLockTuple(scan->heapRelation, heapTuple, scan->xs_snapshot);
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got_heap_tuple = heap_hot_search_buffer(tid, scan->heapRelation,
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scan->xs_cbuf,
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scan->xs_snapshot,
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&scan->xs_ctup,
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&all_dead,
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!scan->xs_continue_hot);
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LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);
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if (got_heap_tuple)
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{
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/*
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* Only in a non-MVCC snapshot can more than one member of the
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* HOT chain be visible.
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*/
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scan->xs_continue_hot = !IsMVCCSnapshot(scan->xs_snapshot);
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pgstat_count_heap_fetch(scan->indexRelation);
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return heapTuple;
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}
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/*
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* If we can't see it, maybe no one else can either. Check to see
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* if the tuple is dead to all transactions. If we find that all
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* the tuples in the HOT chain are dead, we'll signal the index AM
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* to not return that TID on future indexscans.
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*/
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if (scan->xs_hot_dead &&
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HeapTupleSatisfiesVacuum(heapTuple->t_data, RecentGlobalXmin,
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scan->xs_cbuf) != HEAPTUPLE_DEAD)
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scan->xs_hot_dead = false;
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/*
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* Check to see if HOT chain continues past this tuple; if so
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* fetch the next offnum (we don't bother storing it into
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* xs_next_hot, but must store xs_prev_xmax), and loop around.
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*/
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if (HeapTupleIsHotUpdated(heapTuple))
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{
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Assert(ItemPointerGetBlockNumber(ctid) ==
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ItemPointerGetBlockNumber(tid));
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offnum = ItemPointerGetOffsetNumber(ctid);
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at_chain_start = false;
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scan->xs_prev_xmax = HeapTupleHeaderGetXmax(heapTuple->t_data);
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}
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else
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break; /* end of chain */
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} /* loop over a single HOT chain */
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LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);
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/* Loop around to ask index AM for another TID */
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scan->xs_next_hot = InvalidOffsetNumber;
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scan->xs_continue_hot = false;
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}
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/* Release any held pin on a heap page */
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@ -349,9 +349,11 @@ bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres)
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{
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OffsetNumber offnum = tbmres->offsets[curslot];
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ItemPointerData tid;
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HeapTupleData heapTuple;
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ItemPointerSet(&tid, page, offnum);
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if (heap_hot_search_buffer(&tid, scan->rs_rd, buffer, snapshot, NULL))
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if (heap_hot_search_buffer(&tid, scan->rs_rd, buffer, snapshot,
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&heapTuple, NULL, true))
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scan->rs_vistuples[ntup++] = ItemPointerGetOffsetNumber(&tid);
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}
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}
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@ -83,7 +83,8 @@ extern bool heap_fetch(Relation relation, Snapshot snapshot,
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HeapTuple tuple, Buffer *userbuf, bool keep_buf,
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Relation stats_relation);
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extern bool heap_hot_search_buffer(ItemPointer tid, Relation relation,
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Buffer buffer, Snapshot snapshot, bool *all_dead);
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Buffer buffer, Snapshot snapshot, HeapTuple heapTuple,
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bool *all_dead, bool first_call);
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extern bool heap_hot_search(ItemPointer tid, Relation relation,
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Snapshot snapshot, bool *all_dead);
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@ -84,9 +84,7 @@ typedef struct IndexScanDescData
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bool xs_recheck; /* T means scan keys must be rechecked */
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/* state data for traversing HOT chains in index_getnext */
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bool xs_hot_dead; /* T if all members of HOT chain are dead */
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OffsetNumber xs_next_hot; /* next member of HOT chain, if any */
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TransactionId xs_prev_xmax; /* previous HOT chain member's XMAX, if any */
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bool xs_continue_hot; /* T if must keep walking HOT chain */
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} IndexScanDescData;
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/* Struct for heap-or-index scans of system tables */
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