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Adjustments to the btree fastpath optimization.

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This optimization was introduced in commit 2b272734. The changes include
some additional comments and documentation, and also these more
substantive changes:
. ensure the optimization is only applied on the leaf node of a tree
whose root is on level 2 or more. It's of little value on small trees.
. Delay calling RelationSetTargetBlock() until after the critical
section of _bt_insertonpg
. ensure the optimization is also applied to unlogged tables.

Pavan Deolasee and Peter Geoghegan with some very light editing from me.

Discussion: https://postgr.es/m/CABOikdO8jhRarNC60nZLktZYhxt+TK8z_V97+Ny499YQdyAfug@mail.gmail.com
This commit is contained in:
Andrew Dunstan 2018-04-10 18:21:03 -04:00
parent 31f1f0bb4f
commit 074251db67
2 changed files with 71 additions and 11 deletions
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19
src/backend/access/nbtree/README
View File

@ -375,6 +375,25 @@ positives, so long as it never gives a false negative. This makes it
possible to implement the test with a small counter value stored on each
index page.
Fastpath For Index Insertion
----------------------------
We optimize for a common case of insertion of increasing index key
values by caching the last page to which this backend inserted the last
value, if this page was the rightmost leaf page. For the next insert, we
can then quickly check if the cached page is still the rightmost leaf
page and also the correct place to hold the current value. We can avoid
the cost of walking down the tree in such common cases.
The optimization works on the assumption that there can only be one
non-ignorable leaf rightmost page, and so even a RecentGlobalXmin style
interlock isn't required. We cannot fail to detect that our hint was
invalidated, because there can only be one such page in the B-Tree at
any time. It's possible that the page will be deleted and recycled
without a backend's cached page also being detected as invalidated, but
only when we happen to recycle a block that once again gets recycled as the
rightmost leaf page.
On-the-Fly Deletion Of Index Tuples
-----------------------------------

63
src/backend/access/nbtree/nbtinsert.c
View File

@ -26,6 +26,8 @@
#include "storage/smgr.h"
#include "utils/tqual.h"
/* Minimum tree height for application of fastpath optimization */
#define BTREE_FASTPATH_MIN_LEVEL 2
typedef struct
{
@ -125,7 +127,7 @@ _bt_doinsert(Relation rel, IndexTuple itup,
/*
* It's very common to have an index on an auto-incremented or
* monotonically increasing value. In such cases, every insertion happens
* towards the end of the index. We try to optimise that case by caching
* towards the end of the index. We try to optimize that case by caching
* the right-most leaf of the index. If our cached block is still the
* rightmost leaf, has enough free space to accommodate a new entry and
* the insertion key is strictly greater than the first key in this page,
@ -176,13 +178,17 @@ top:
* the first key on the page.
*/
if (P_ISLEAF(lpageop) && P_RIGHTMOST(lpageop) &&
!P_INCOMPLETE_SPLIT(lpageop) &&
!P_IGNORE(lpageop) &&
(PageGetFreeSpace(page) > itemsz) &&
PageGetMaxOffsetNumber(page) >= P_FIRSTDATAKEY(lpageop) &&
_bt_compare(rel, indnkeyatts, itup_scankey, page,
P_FIRSTDATAKEY(lpageop)) > 0)
{
/*
* The right-most block should never have incomplete split. But
* be paranoid and check for it anyway.
*/
Assert(!P_INCOMPLETE_SPLIT(lpageop));
fastpath = true;
}
else
@ -868,6 +874,24 @@ _bt_insertonpg(Relation rel,
bool newitemonleft;
Buffer rbuf;
/*
* If we're here then a pagesplit is needed. We should never reach here
* if we're using the fastpath since we should have checked for all the
* required conditions, including the fact that this page has enough
* freespace. Note that this routine can in theory deal with the
* situation where a NULL stack pointer is passed (that's what would
* happen if the fastpath is taken), like it does during crash
* recovery. But that path is much slower, defeating the very purpose
* of the optimization. The following assertion should protect us from
* any future code changes that invalidate those assumptions.
*
* Note that whenever we fail to take the fastpath, we clear the
* cached block. Checking for a valid cached block at this point is
* enough to decide whether we're in a fastpath or not.
*/
Assert(!(P_ISLEAF(lpageop) &&
BlockNumberIsValid(RelationGetTargetBlock(rel))));
/* Choose the split point */
firstright = _bt_findsplitloc(rel, page,
newitemoff, itemsz,
@ -905,6 +929,7 @@ _bt_insertonpg(Relation rel,
BTMetaPageData *metad = NULL;
OffsetNumber itup_off;
BlockNumber itup_blkno;
BlockNumber cachedBlock = InvalidBlockNumber;
itup_off = newitemoff;
itup_blkno = BufferGetBlockNumber(buf);
@ -962,6 +987,15 @@ _bt_insertonpg(Relation rel,
MarkBufferDirty(cbuf);
}
/*
* Cache the block information if we just inserted into the rightmost
* leaf page of the index and it's not the root page. For very small
* index where root is also the leaf, there is no point trying for any
* optimization.
*/
if (P_RIGHTMOST(lpageop) && P_ISLEAF(lpageop) && !P_ISROOT(lpageop))
cachedBlock = BufferGetBlockNumber(buf);
/* XLOG stuff */
if (RelationNeedsWAL(rel))
{
@ -977,16 +1011,7 @@ _bt_insertonpg(Relation rel,
XLogRegisterData((char *) &xlrec, SizeOfBtreeInsert);
if (P_ISLEAF(lpageop))
{
xlinfo = XLOG_BTREE_INSERT_LEAF;
/*
* Cache the block information if we just inserted into the
* rightmost leaf page of the index.
*/
if (P_RIGHTMOST(lpageop))
RelationSetTargetBlock(rel, BufferGetBlockNumber(buf));
}
else
{
/*
@ -1048,6 +1073,22 @@ _bt_insertonpg(Relation rel,
if (BufferIsValid(cbuf))
_bt_relbuf(rel, cbuf);
_bt_relbuf(rel, buf);
/*
* If we decided to cache the insertion target block, then set it now.
* But before that, check for the height of the tree and don't go for
* the optimization for small indexes. We defer that check to this
* point to ensure that we don't call _bt_getrootheight while holding
* lock on any other block.
*
* We do this after dropping locks on all buffers. So the information
* about whether the insertion block is still the rightmost block or
* not may have changed in between. But we will deal with that during
* next insert operation. No special care is required while setting it.
*/
if (BlockNumberIsValid(cachedBlock) &&
_bt_getrootheight(rel) >= BTREE_FASTPATH_MIN_LEVEL)
RelationSetTargetBlock(rel, cachedBlock);
}
}