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Commit39eabec

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nbtree: Move fastpath NULL descent stack assertion.
Commit074251d added an assertion that verified the fastpath/rightmostpage insert optimization's assumption about free space: There shouldalways be enough free space on the page to insert the new item withoutsplitting the page. Otherwise, we end up using the "concurrent rootpage split" phony/fake stack path in _bt_insert_parent(). This does notlead to incorrect behavior, but it is likely to be far slower thansimply using the regular _bt_search() path. The assertion catchesserious performance bugs that would probably take a long time to detectany other way.It seems much more natural to make this assertion just before the pointthat we generate a fake/phony descent stack. Move the assert there.This also makes _bt_insertonpg() a bit more readable.
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‎src/backend/access/nbtree/nbtinsert.c

Lines changed: 20 additions & 26 deletions
Original file line numberDiff line numberDiff line change
@@ -174,11 +174,13 @@ _bt_doinsert(Relation rel, IndexTuple itup,
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/*
175175
* Check if the page is still the rightmost leaf page, has enough
176176
* free space to accommodate the new tuple, and the insertion scan
177-
* key is strictly greater than the first key on the page.
177+
* key is strictly greater than the first key on the page. Note
178+
* that _bt_insert_parent() has an assertion that catches leaf
179+
* page splits that somehow follow from a fastpath insert.
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*/
179181
if (P_ISLEAF(lpageop)&&P_RIGHTMOST(lpageop)&&
180182
!P_IGNORE(lpageop)&&
181-
(PageGetFreeSpace(page)>insertstate.itemsz)&&
183+
PageGetFreeSpace(page)>insertstate.itemsz&&
182184
PageGetMaxOffsetNumber(page) >=P_FIRSTDATAKEY(lpageop)&&
183185
_bt_compare(rel,itup_key,page,P_FIRSTDATAKEY(lpageop))>0)
184186
{
@@ -1140,24 +1142,6 @@ _bt_insertonpg(Relation rel,
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boolis_only=P_LEFTMOST(lpageop)&&P_RIGHTMOST(lpageop);
11411143
Bufferrbuf;
11421144

1143-
/*
1144-
* If we're here then a pagesplit is needed. We should never reach
1145-
* here if we're using the fastpath since we should have checked for
1146-
* all the required conditions, including the fact that this page has
1147-
* enough freespace. Note that this routine can in theory deal with
1148-
* the situation where a NULL stack pointer is passed (that's what
1149-
* would happen if the fastpath is taken). But that path is much
1150-
* slower, defeating the very purpose of the optimization. The
1151-
* following assertion should protect us from any future code changes
1152-
* that invalidate those assumptions.
1153-
*
1154-
* Note that whenever we fail to take the fastpath, we clear the
1155-
* cached block. Checking for a valid cached block at this point is
1156-
* enough to decide whether we're in a fastpath or not.
1157-
*/
1158-
Assert(!(P_ISLEAF(lpageop)&&
1159-
BlockNumberIsValid(RelationGetTargetBlock(rel))));
1160-
11611145
/* split the buffer into left and right halves */
11621146
rbuf=_bt_split(rel,itup_key,buf,cbuf,newitemoff,itemsz,itup,
11631147
origitup,nposting,postingoff);
@@ -1370,12 +1354,6 @@ _bt_insertonpg(Relation rel,
13701354
* the optimization for small indexes. We defer that check to this
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* point to ensure that we don't call _bt_getrootheight while holding
13721356
* lock on any other block.
1373-
*
1374-
* We do this after dropping locks on all buffers. So the information
1375-
* about whether the insertion block is still the rightmost block or
1376-
* not may have changed in between. But we will deal with that during
1377-
* next insert operation. No special care is required while setting
1378-
* it.
13791357
*/
13801358
if (BlockNumberIsValid(cachedBlock)&&
13811359
_bt_getrootheight(rel) >=BTREE_FASTPATH_MIN_LEVEL)
@@ -2066,6 +2044,22 @@ _bt_insert_parent(Relation rel,
20662044

20672045
elog(DEBUG2,"concurrent ROOT page split");
20682046
lpageop= (BTPageOpaque)PageGetSpecialPointer(page);
2047+
2048+
/*
2049+
* We should never reach here when a leaf page split takes place
2050+
* despite the insert of newitem being able to apply the fastpath
2051+
* optimization. Make sure of that with an assertion.
2052+
*
2053+
* This is more of a performance issue than a correctness issue.
2054+
* The fastpath won't have a descent stack. Using a phony stack
2055+
* here works, but never rely on that. The fastpath should be
2056+
* rejected when the rightmost leaf page will split, since it's
2057+
* faster to go through _bt_search() and get a stack in the usual
2058+
* way.
2059+
*/
2060+
Assert(!(P_ISLEAF(lpageop)&&
2061+
BlockNumberIsValid(RelationGetTargetBlock(rel))));
2062+
20692063
/* Find the leftmost page at the next level up */
20702064
pbuf=_bt_get_endpoint(rel,lpageop->btpo.level+1, false,
20712065
NULL);

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