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raw | patch | inline | side by side (parent: a2b1faa)
| author | Peter Geoghegan <pg@bowt.ie> | |
| Wed, 18 Mar 2020 21:42:49 +0000 (14:42 -0700) | ||
| committer | Peter Geoghegan <pg@bowt.ie> | |
| Wed, 18 Mar 2020 21:42:49 +0000 (14:42 -0700) |
Commit 2b272734, which added the fastpath rightmost leaf page cache
insert optimization, added code to _bt_doinsert() to handle using and
invalidating the backend local block cache. It doesn't seem like a good
place to handle these low level details, though. _bt_doinsert() is
supposed to be a high level function -- it is the main entry point to
nbtinsert.c.
Restructure the code by placing handling of the rightmost block cache at
the start of a new _bt_search() shim function, _bt_search_insert(). The
new function is called from _bt_doinsert(), which uses it as a
_bt_search() variant that conveniently accepts its BTInsertState state
as an argument. _bt_doinsert() no longer needs to directly consider the
fastpath optimization.
Discussion: https://postgr.es/m/CAH2-Wzk59cxKJRd=rfbyub6-V4yWRjsOYRkUNHBLT1P1GdtCQQ@mail.gmail.com
insert optimization, added code to _bt_doinsert() to handle using and
invalidating the backend local block cache. It doesn't seem like a good
place to handle these low level details, though. _bt_doinsert() is
supposed to be a high level function -- it is the main entry point to
nbtinsert.c.
Restructure the code by placing handling of the rightmost block cache at
the start of a new _bt_search() shim function, _bt_search_insert(). The
new function is called from _bt_doinsert(), which uses it as a
_bt_search() variant that conveniently accepts its BTInsertState state
as an argument. _bt_doinsert() no longer needs to directly consider the
fastpath optimization.
Discussion: https://postgr.es/m/CAH2-Wzk59cxKJRd=rfbyub6-V4yWRjsOYRkUNHBLT1P1GdtCQQ@mail.gmail.com
| src/backend/access/nbtree/nbtinsert.c | patch | blob | blame | history |
index 966c0bb532fe65558209f357c09b3fc2c673ebb1..bb19c3d26370fdbc59558a9faf9bce1638a74303 100644 (file)
#define BTREE_FASTPATH_MIN_LEVEL 2
+static BTStack _bt_search_insert(Relation rel, BTInsertState insertstate);
static TransactionId _bt_check_unique(Relation rel, BTInsertState insertstate,
Relation heapRel,
IndexUniqueCheck checkUnique, bool *is_unique,
bool is_unique = false;
BTInsertStateData insertstate;
BTScanInsert itup_key;
- BTStack stack = NULL;
- Buffer buf;
- bool fastpath;
+ BTStack stack;
bool checkingunique = (checkUnique != UNIQUE_CHECK_NO);
/* we need an insertion scan key to do our search, so build one */
insertstate.buf = InvalidBuffer;
insertstate.postingoff = 0;
+search:
+
/*
- * 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 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,
- * then we can safely conclude that the new key will be inserted in the
- * cached block. So we simply search within the cached block and insert
- * the key at the appropriate location. We call it a fastpath.
- *
- * Testing has revealed, though, that the fastpath can result in increased
- * contention on the exclusive-lock on the rightmost leaf page. So we
- * conditionally check if the lock is available. If it's not available
- * then we simply abandon the fastpath and take the regular path. This
- * makes sense because unavailability of the lock also signals that some
- * other backend might be concurrently inserting into the page, thus
- * reducing our chances to finding an insertion place in this page.
+ * Find and lock the leaf page that the tuple should be added to by
+ * searching from the root page. insertstate.buf will hold a buffer that
+ * is locked in exclusive mode afterwards.
*/
-top:
- fastpath = false;
- if (RelationGetTargetBlock(rel) != InvalidBlockNumber)
- {
- Page page;
- BTPageOpaque lpageop;
-
- /*
- * Conditionally acquire exclusive lock on the buffer before doing any
- * checks. If we don't get the lock, we simply follow slowpath. If we
- * do get the lock, this ensures that the index state cannot change,
- * as far as the rightmost part of the index is concerned.
- */
- buf = ReadBuffer(rel, RelationGetTargetBlock(rel));
-
- if (ConditionalLockBuffer(buf))
- {
- _bt_checkpage(rel, buf);
-
- page = BufferGetPage(buf);
-
- lpageop = (BTPageOpaque) PageGetSpecialPointer(page);
-
- /*
- * Check if the page is still the rightmost leaf page, has enough
- * free space to accommodate the new tuple, and the insertion scan
- * key is strictly greater than the first key on the page. Note
- * that _bt_insert_parent() has an assertion that catches leaf
- * page splits that somehow follow from a fastpath insert.
- */
- if (P_ISLEAF(lpageop) && P_RIGHTMOST(lpageop) &&
- !P_IGNORE(lpageop) &&
- PageGetFreeSpace(page) > insertstate.itemsz &&
- PageGetMaxOffsetNumber(page) >= P_FIRSTDATAKEY(lpageop) &&
- _bt_compare(rel, itup_key, page, P_FIRSTDATAKEY(lpageop)) > 0)
- {
- fastpath = true;
- }
- else
- {
- _bt_relbuf(rel, buf);
-
- /*
- * Something did not work out. Just forget about the cached
- * block and follow the normal path. It might be set again if
- * the conditions are favourable.
- */
- RelationSetTargetBlock(rel, InvalidBlockNumber);
- }
- }
- else
- {
- ReleaseBuffer(buf);
-
- /*
- * If someone's holding a lock, it's likely to change anyway, so
- * don't try again until we get an updated rightmost leaf.
- */
- RelationSetTargetBlock(rel, InvalidBlockNumber);
- }
- }
-
- if (!fastpath)
- {
- /*
- * Find the first page containing this key. Buffer returned by
- * _bt_search() is locked in exclusive mode.
- */
- stack = _bt_search(rel, itup_key, &buf, BT_WRITE, NULL);
- }
-
- insertstate.buf = buf;
- buf = InvalidBuffer; /* insertstate.buf now owns the buffer */
+ stack = _bt_search_insert(rel, &insertstate);
/*
- * If we're not allowing duplicates, make sure the key isn't already in
- * the index.
+ * checkingunique inserts are not allowed to go ahead when two tuples with
+ * equal key attribute values would be visible to new MVCC snapshots once
+ * the xact commits. Check for conflicts in the locked page/buffer (if
+ * needed) here.
+ *
+ * It might be necessary to check a page to the right in _bt_check_unique,
+ * though that should be very rare. In practice the first page the value
+ * could be on (with scantid omitted) is almost always also the only page
+ * that a matching tuple might be found on. This is due to the behavior
+ * of _bt_findsplitloc with duplicate tuples -- a group of duplicates can
+ * only be allowed to cross a page boundary when there is no candidate
+ * leaf page split point that avoids it. Also, _bt_check_unique can use
+ * the leaf page high key to determine that there will be no duplicates on
+ * the right sibling without actually visiting it (it uses the high key in
+ * cases where the new item happens to belong at the far right of the leaf
+ * page).
*
* NOTE: obviously, _bt_check_unique can only detect keys that are already
* in the index; so it cannot defend against concurrent insertions of the
* insertion. (This requires some care in _bt_findinsertloc.)
*
* If we must wait for another xact, we release the lock while waiting,
- * and then must start over completely.
+ * and then must perform a new search.
*
* For a partial uniqueness check, we don't wait for the other xact. Just
* let the tuple in and return false for possibly non-unique, or true for
xwait = _bt_check_unique(rel, &insertstate, heapRel, checkUnique,
&is_unique, &speculativeToken);
- if (TransactionIdIsValid(xwait))
+ if (unlikely(TransactionIdIsValid(xwait)))
{
/* Have to wait for the other guy ... */
_bt_relbuf(rel, insertstate.buf);
/* start over... */
if (stack)
_bt_freestack(stack);
- goto top;
+ goto search;
}
/* Uniqueness is established -- restore heap tid as scantid */
return is_unique;
}
+/*
+ * _bt_search_insert() -- _bt_search() wrapper for inserts
+ *
+ * Search the tree for a particular scankey, or more precisely for the first
+ * leaf page it could be on. Try to make use of the fastpath optimization's
+ * rightmost leaf page cache before actually searching the tree from the root
+ * page, though.
+ *
+ * Return value is a stack of parent-page pointers (though see notes about
+ * fastpath optimization and page splits below). insertstate->buf is set to
+ * the address of the leaf-page buffer, which is write-locked and pinned in
+ * all cases (if necessary by creating a new empty root page for caller).
+ *
+ * The fastpath optimization avoids most of the work of searching the tree
+ * repeatedly when a single backend inserts successive new tuples on the
+ * rightmost leaf page of an index. A backend cache of the rightmost leaf
+ * page is maintained within _bt_insertonpg(), and used here. The cache is
+ * invalidated here when an insert of a non-pivot tuple must take place on a
+ * non-rightmost leaf page.
+ *
+ * The optimization helps with indexes on an auto-incremented field. It also
+ * helps with indexes on datetime columns, as well as indexes with lots of
+ * NULL values. (NULLs usually get inserted in the rightmost page for single
+ * column indexes, since they usually get treated as coming after everything
+ * else in the key space. Individual NULL tuples will generally be placed on
+ * the rightmost leaf page due to the influence of the heap TID column.)
+ *
+ * Note that we avoid applying the optimization when there is insufficient
+ * space on the rightmost page to fit caller's new item. This is necessary
+ * because we'll need to return a real descent stack when a page split is
+ * expected (actually, caller can cope with a leaf page split that uses a NULL
+ * stack, but that's very slow and so must be avoided). Note also that the
+ * fastpath optimization acquires the lock on the page conditionally as a way
+ * of reducing extra contention when there are concurrent insertions into the
+ * rightmost page (we give up if we'd have to wait for the lock). We assume
+ * that it isn't useful to apply the optimization when there is contention,
+ * since each per-backend cache won't stay valid for long.
+ */
+static BTStack
+_bt_search_insert(Relation rel, BTInsertState insertstate)
+{
+ Assert(insertstate->buf == InvalidBuffer);
+ Assert(!insertstate->bounds_valid);
+ Assert(insertstate->postingoff == 0);
+
+ if (RelationGetTargetBlock(rel) != InvalidBlockNumber)
+ {
+ /* Simulate a _bt_getbuf() call with conditional locking */
+ insertstate->buf = ReadBuffer(rel, RelationGetTargetBlock(rel));
+ if (ConditionalLockBuffer(insertstate->buf))
+ {
+ Page page;
+ BTPageOpaque lpageop;
+
+ _bt_checkpage(rel, insertstate->buf);
+ page = BufferGetPage(insertstate->buf);
+ lpageop = (BTPageOpaque) PageGetSpecialPointer(page);
+
+ /*
+ * Check if the page is still the rightmost leaf page and has
+ * enough free space to accommodate the new tuple. Also check
+ * that the insertion scan key is strictly greater than the first
+ * non-pivot tuple on the page. (Note that we expect itup_key's
+ * scantid to be unset when our caller is a checkingunique
+ * inserter.)
+ */
+ if (P_RIGHTMOST(lpageop) &&
+ P_ISLEAF(lpageop) &&
+ !P_IGNORE(lpageop) &&
+ PageGetFreeSpace(page) > insertstate->itemsz &&
+ PageGetMaxOffsetNumber(page) >= P_HIKEY &&
+ _bt_compare(rel, insertstate->itup_key, page, P_HIKEY) > 0)
+ {
+ /*
+ * Caller can use the fastpath optimization because cached
+ * block is still rightmost leaf page, which can fit caller's
+ * new tuple without splitting. Keep block in local cache for
+ * next insert, and have caller use NULL stack.
+ *
+ * Note that _bt_insert_parent() has an assertion that catches
+ * leaf page splits that somehow follow from a fastpath insert
+ * (it should only be passed a NULL stack when it must deal
+ * with a concurrent root page split, and never because a NULL
+ * stack was returned here).
+ */
+ return NULL;
+ }
+
+ /* Page unsuitable for caller, drop lock and pin */
+ _bt_relbuf(rel, insertstate->buf);
+ }
+ else
+ {
+ /* Lock unavailable, drop pin */
+ ReleaseBuffer(insertstate->buf);
+ }
+
+ /* Forget block, since cache doesn't appear to be useful */
+ RelationSetTargetBlock(rel, InvalidBlockNumber);
+ }
+
+ /* Cannot use optimization -- descend tree, return proper descent stack */
+ return _bt_search(rel, insertstate->itup_key, &insertstate->buf, BT_WRITE,
+ NULL);
+}
+
/*
* _bt_check_unique() -- Check for violation of unique index constraint
*
}
else
{
+ bool isleaf = P_ISLEAF(lpageop);
+ bool isrightmost = P_RIGHTMOST(lpageop);
Buffer metabuf = InvalidBuffer;
Page metapg = NULL;
BTMetaPageData *metad = NULL;
- BlockNumber cachedBlock = InvalidBlockNumber;
+ BlockNumber blockcache;
/*
* If we are doing this insert because we split a page that was the
*/
if (split_only_page)
{
- Assert(!P_ISLEAF(lpageop));
+ Assert(!isleaf);
+ Assert(BufferIsValid(cbuf));
metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
metapg = BufferGetPage(metabuf);
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))
{
XLogBeginInsert();
XLogRegisterData((char *) &xlrec, SizeOfBtreeInsert);
- if (P_ISLEAF(lpageop) && postingoff == 0)
+ if (isleaf && postingoff == 0)
{
/* Simple leaf insert */
xlinfo = XLOG_BTREE_INSERT_LEAF;
recptr = XLogInsert(RM_BTREE_ID, xlinfo);
if (BufferIsValid(metabuf))
- {
PageSetLSN(metapg, recptr);
- }
if (BufferIsValid(cbuf))
- {
PageSetLSN(BufferGetPage(cbuf), recptr);
- }
PageSetLSN(page, recptr);
}
END_CRIT_SECTION();
- /* release buffers */
+ /* Release subsidiary buffers */
if (BufferIsValid(metabuf))
_bt_relbuf(rel, metabuf);
if (BufferIsValid(cbuf))
_bt_relbuf(rel, cbuf);
+
+ /*
+ * Cache the block number if this is the rightmost leaf page. Cache
+ * may be used by a future inserter within _bt_search_insert().
+ */
+ blockcache = InvalidBlockNumber;
+ if (isrightmost && isleaf && !P_ISROOT(lpageop))
+ blockcache = BufferGetBlockNumber(buf);
+
+ /* Release buffer for insertion target block */
_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.
+ * If we decided to cache the insertion target block before releasing
+ * its buffer lock, then cache it now. Check the height of the tree
+ * first, though. We don't go for the optimization with small
+ * indexes. Defer final check to this point to ensure that we don't
+ * call _bt_getrootheight while holding a buffer lock.
*/
- if (BlockNumberIsValid(cachedBlock) &&
+ if (BlockNumberIsValid(blockcache) &&
_bt_getrootheight(rel) >= BTREE_FASTPATH_MIN_LEVEL)
- RelationSetTargetBlock(rel, cachedBlock);
+ RelationSetTargetBlock(rel, blockcache);
}
/* be tidy */
* This is more of a performance issue than a correctness issue.
* The fastpath won't have a descent stack. Using a phony stack
* here works, but never rely on that. The fastpath should be
- * rejected when the rightmost leaf page will split, since it's
- * faster to go through _bt_search() and get a stack in the usual
- * way.
+ * rejected within _bt_search_insert() when the rightmost leaf
+ * page will split, since it's faster to go through _bt_search()
+ * and get a stack in the usual way.
*/
Assert(!(P_ISLEAF(lpageop) &&
BlockNumberIsValid(RelationGetTargetBlock(rel))));