On Thu, Oct 10, 2024 at 1:41 PM Peter Geoghegan <p...@bowt.ie> wrote:
> The main simplification new to v4 is that v4 isolates the need to call
> _bt_drop_lock_and_maybe_pin to only 2 functions: _bt_readnextpage, and
> its new _bt_readfirstpage "sibling" function. The functions have
> similar preconditions, and identical postconditions -- all of which
> are now clearly documented.
Worked on this a little more today. Attached is v5, which goes even
further than v4 did. Highlights:
* Completely gets rid of _bt_initialize_more_data by moving its code
into the new _bt_readfirstpage function.
* Adds similar "initialize moreLeft/moreRight" code to the
corresponding point within _bt_readnextpage (that is, at the start of
_bt_readnextpage) -- meaning I moved a bit more code from _bt_steppage
into _bt_readnextpage.
Again, _bt_readnextpage and the new _bt_readfirstpage function are
intended to be "sibling" functions (while _bt_steppage is demoted to a
helper that sets up a call to _bt_readfirstpage). The matching
handling of currPos initialization within _bt_readnextpage and the
_bt_readfirstpage pushes this further than in v4.
* We now reset currPos state (including even its moreLeft/moreRight
fields) within _bt_parallel_seize, automatically and regardless of any
other details.
It doesn't really make sense that (up until now) we've trusted the
shared state that we seized from the parallel scan to kinda be in sync
with the local currPos state (at least its moreLeft/moreRight need to
be in sync, to avoid confusion within _bt_readnextpage). This allowed
me to remove several parallel-only BTScanPosInvalidate calls from
_bt_readnextpage.
This "currPos must be kept in sync with parallel scan state" confusion
is exemplified by the following code snippet (which FWIW was already
removed by earlier revisions of the patch), that appears in the
backwards scan block of _bt_readnextpage on master:
"""
/*
* Should only happen in parallel cases, when some other backend
* advanced the scan.
*/
if (so->currPos.currPage != blkno)
{
BTScanPosUnpinIfPinned(so->currPos);
so->currPos.currPage = blkno;
}
"""
Why should the parallel scan have any concern for what currPos is, in general?
* Now nbtree has only one PredicateLockPage call, inside _bt_readpage.
This saves us an extra BufferGetBlockNumber call. (v4 pushed
PredicateLockPage calls down one layer, v5 pushes them down another
layer still.)
* Improved precondition/postcondition comments, and generally cleaner
separation between _bt_steppage and _bt_readnextpage.
--
Peter Geoghegan
From b3c892d428688057378c5a74e2bb313c1a6b4c2e Mon Sep 17 00:00:00 2001
From: Peter Geoghegan <pg@bowt.ie>
Date: Tue, 8 Oct 2024 11:40:54 -0400
Subject: [PATCH v5] Optimize and simplify nbtree backwards scans.
Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAEze2WgpBGRgTTxTWVPXc9+PB6fc1a7t+VyGXHzfnrFXcQVxnA@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-WzkBTuFv7W2+84jJT8mWZLXVL0GHq2hMUTn6c9Vw=eYrCw@mail.gmail.com
---
src/include/access/nbtree.h | 21 +-
src/backend/access/nbtree/nbtree.c | 49 ++-
src/backend/access/nbtree/nbtsearch.c | 603 +++++++++++++-------------
src/backend/access/nbtree/nbtutils.c | 2 +-
4 files changed, 347 insertions(+), 328 deletions(-)
diff --git a/src/include/access/nbtree.h b/src/include/access/nbtree.h
index d64300fb9..594eddaec 100644
--- a/src/include/access/nbtree.h
+++ b/src/include/access/nbtree.h
@@ -954,6 +954,7 @@ typedef struct BTScanPosData
XLogRecPtr lsn; /* pos in the WAL stream when page was read */
BlockNumber currPage; /* page referenced by items array */
+ BlockNumber prevPage; /* page's left link when we scanned it */
BlockNumber nextPage; /* page's right link when we scanned it */
/*
@@ -965,15 +966,6 @@ typedef struct BTScanPosData
bool moreLeft;
bool moreRight;
- /*
- * Direction of the scan at the time that _bt_readpage was called.
- *
- * Used by btrestrpos to "restore" the scan's array keys by resetting each
- * array to its first element's value (first in this scan direction). This
- * avoids the need to directly track the array keys in btmarkpos.
- */
- ScanDirection dir;
-
/*
* If we are doing an index-only scan, nextTupleOffset is the first free
* location in the associated tuple storage workspace.
@@ -1022,6 +1014,7 @@ typedef BTScanPosData *BTScanPos;
#define BTScanPosInvalidate(scanpos) \
do { \
(scanpos).currPage = InvalidBlockNumber; \
+ (scanpos).prevPage = InvalidBlockNumber; \
(scanpos).nextPage = InvalidBlockNumber; \
(scanpos).buf = InvalidBuffer; \
(scanpos).lsn = InvalidXLogRecPtr; \
@@ -1071,6 +1064,7 @@ typedef struct BTScanOpaqueData
* determine if there is a mark, first look at markItemIndex, then at
* markPos.
*/
+ ScanDirection markDir; /* direction of scan with mark */
int markItemIndex; /* itemIndex, or -1 if not valid */
/* keep these last in struct for efficiency */
@@ -1090,7 +1084,6 @@ typedef struct BTReadPageState
OffsetNumber minoff; /* Lowest non-pivot tuple's offset */
OffsetNumber maxoff; /* Highest non-pivot tuple's offset */
IndexTuple finaltup; /* Needed by scans with array keys */
- BlockNumber prev_scan_page; /* previous _bt_parallel_release block */
Page page; /* Page being read */
/* Per-tuple input parameters, set by _bt_readpage for _bt_checkkeys */
@@ -1192,11 +1185,13 @@ extern int btgettreeheight(Relation rel);
* prototypes for internal functions in nbtree.c
*/
extern bool _bt_parallel_seize(IndexScanDesc scan, BlockNumber *pageno,
- bool first);
-extern void _bt_parallel_release(IndexScanDesc scan, BlockNumber scan_page);
+ BlockNumber *currblkno, bool first);
+extern void _bt_parallel_release(IndexScanDesc scan,
+ BlockNumber next_scan_page,
+ BlockNumber curr_page);
extern void _bt_parallel_done(IndexScanDesc scan);
extern void _bt_parallel_primscan_schedule(IndexScanDesc scan,
- BlockNumber prev_scan_page);
+ BlockNumber curr_page);
/*
* prototypes for functions in nbtdedup.c
diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index 56e502c4f..db20b1138 100644
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -66,7 +66,9 @@ typedef enum
*/
typedef struct BTParallelScanDescData
{
- BlockNumber btps_scanPage; /* latest or next page to be scanned */
+ BlockNumber btps_nextScanPage; /* next page to be scanned */
+ BlockNumber btps_lastCurrPage; /* page whose sibling link was copied into
+ * btps_scanPage */
BTPS_State btps_pageStatus; /* indicates whether next page is
* available for scan. see above for
* possible states of parallel scan. */
@@ -520,7 +522,7 @@ btrestrpos(IndexScanDesc scan)
/* Reset the scan's array keys (see _bt_steppage for why) */
if (so->numArrayKeys)
{
- _bt_start_array_keys(scan, so->currPos.dir);
+ _bt_start_array_keys(scan, so->markDir);
so->needPrimScan = false;
}
}
@@ -548,7 +550,8 @@ btinitparallelscan(void *target)
BTParallelScanDesc bt_target = (BTParallelScanDesc) target;
SpinLockInit(&bt_target->btps_mutex);
- bt_target->btps_scanPage = InvalidBlockNumber;
+ bt_target->btps_nextScanPage = InvalidBlockNumber;
+ bt_target->btps_lastCurrPage = InvalidBlockNumber;
bt_target->btps_pageStatus = BTPARALLEL_NOT_INITIALIZED;
ConditionVariableInit(&bt_target->btps_cv);
}
@@ -573,7 +576,8 @@ btparallelrescan(IndexScanDesc scan)
* consistency.
*/
SpinLockAcquire(&btscan->btps_mutex);
- btscan->btps_scanPage = InvalidBlockNumber;
+ btscan->btps_nextScanPage = InvalidBlockNumber;
+ btscan->btps_lastCurrPage = InvalidBlockNumber;
btscan->btps_pageStatus = BTPARALLEL_NOT_INITIALIZED;
SpinLockRelease(&btscan->btps_mutex);
}
@@ -600,7 +604,8 @@ btparallelrescan(IndexScanDesc scan)
* Callers should ignore the value of pageno if the return value is false.
*/
bool
-_bt_parallel_seize(IndexScanDesc scan, BlockNumber *pageno, bool first)
+_bt_parallel_seize(IndexScanDesc scan, BlockNumber *pageno,
+ BlockNumber *currblkno, bool first)
{
BTScanOpaque so = (BTScanOpaque) scan->opaque;
bool exit_loop = false;
@@ -609,6 +614,14 @@ _bt_parallel_seize(IndexScanDesc scan, BlockNumber *pageno, bool first)
BTParallelScanDesc btscan;
*pageno = P_NONE;
+ *currblkno = InvalidBlockNumber;
+
+ /*
+ * Reset moreLeft/moreRight. We don't want to retain information from the
+ * last time the backend seized the scan.
+ */
+ BTScanPosInvalidate(so->currPos);
+ so->currPos.moreLeft = so->currPos.moreRight = true;
if (first)
{
@@ -684,7 +697,8 @@ _bt_parallel_seize(IndexScanDesc scan, BlockNumber *pageno, bool first)
* of advancing it to a new page!
*/
btscan->btps_pageStatus = BTPARALLEL_ADVANCING;
- *pageno = btscan->btps_scanPage;
+ *pageno = btscan->btps_nextScanPage;
+ *currblkno = btscan->btps_lastCurrPage;
exit_loop = true;
}
SpinLockRelease(&btscan->btps_mutex);
@@ -699,17 +713,22 @@ _bt_parallel_seize(IndexScanDesc scan, BlockNumber *pageno, bool first)
/*
* _bt_parallel_release() -- Complete the process of advancing the scan to a
- * new page. We now have the new value btps_scanPage; some other backend
+ * new page. We now have the new value btps_nextScanPage; another backend
* can now begin advancing the scan.
*
- * Callers whose scan uses array keys must save their scan_page argument so
+ * Callers whose scan uses array keys must save their curr_page argument so
* that it can be passed to _bt_parallel_primscan_schedule, should caller
* determine that another primitive index scan is required. If that happens,
* scan_page won't be scanned by any backend (unless the next primitive index
* scan lands on scan_page).
+ *
+ * Note: unlike the serial case, parallel scans don't need to remember both
+ * sibling links. Caller can just pass us whichever link is next for the scan
+ * direction because the direction cannot change when parallelism is in use.
*/
void
-_bt_parallel_release(IndexScanDesc scan, BlockNumber scan_page)
+_bt_parallel_release(IndexScanDesc scan, BlockNumber next_scan_page,
+ BlockNumber curr_page)
{
ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
BTParallelScanDesc btscan;
@@ -718,7 +737,8 @@ _bt_parallel_release(IndexScanDesc scan, BlockNumber scan_page)
parallel_scan->ps_offset);
SpinLockAcquire(&btscan->btps_mutex);
- btscan->btps_scanPage = scan_page;
+ btscan->btps_nextScanPage = next_scan_page;
+ btscan->btps_lastCurrPage = curr_page;
btscan->btps_pageStatus = BTPARALLEL_IDLE;
SpinLockRelease(&btscan->btps_mutex);
ConditionVariableSignal(&btscan->btps_cv);
@@ -774,13 +794,13 @@ _bt_parallel_done(IndexScanDesc scan)
/*
* _bt_parallel_primscan_schedule() -- Schedule another primitive index scan.
*
- * Caller passes the block number most recently passed to _bt_parallel_release
+ * Caller passes the curr_page most recently passed to _bt_parallel_release
* by its backend. Caller successfully schedules the next primitive index scan
* if the shared parallel state hasn't been seized since caller's backend last
* advanced the scan.
*/
void
-_bt_parallel_primscan_schedule(IndexScanDesc scan, BlockNumber prev_scan_page)
+_bt_parallel_primscan_schedule(IndexScanDesc scan, BlockNumber curr_page)
{
BTScanOpaque so = (BTScanOpaque) scan->opaque;
ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
@@ -792,10 +812,11 @@ _bt_parallel_primscan_schedule(IndexScanDesc scan, BlockNumber prev_scan_page)
parallel_scan->ps_offset);
SpinLockAcquire(&btscan->btps_mutex);
- if (btscan->btps_scanPage == prev_scan_page &&
+ if (btscan->btps_lastCurrPage == curr_page &&
btscan->btps_pageStatus == BTPARALLEL_IDLE)
{
- btscan->btps_scanPage = InvalidBlockNumber;
+ btscan->btps_nextScanPage = InvalidBlockNumber;
+ btscan->btps_lastCurrPage = InvalidBlockNumber;
btscan->btps_pageStatus = BTPARALLEL_NEED_PRIMSCAN;
/* Serialize scan's current array keys */
diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index fff7c89ea..cd9092130 100644
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -43,12 +43,13 @@ static inline void _bt_savepostingitem(BTScanOpaque so, int itemIndex,
OffsetNumber offnum,
ItemPointer heapTid, int tupleOffset);
static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
-static bool _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir);
-static bool _bt_parallel_readpage(IndexScanDesc scan, BlockNumber blkno,
- ScanDirection dir);
-static Buffer _bt_walk_left(Relation rel, Buffer buf);
+static bool _bt_readfirstpage(IndexScanDesc scan, OffsetNumber offnum,
+ ScanDirection dir);
+static bool _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
+ BlockNumber lastcurrblkno, ScanDirection dir);
+static Buffer _bt_lock_and_validate_left(Relation rel, BlockNumber *blkno,
+ BlockNumber lastcurrblkno);
static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
-static inline void _bt_initialize_more_data(BTScanOpaque so, ScanDirection dir);
/*
@@ -889,7 +890,6 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
ScanKeyData notnullkeys[INDEX_MAX_KEYS];
int keysz = 0;
int i;
- bool status;
StrategyNumber strat_total;
BTScanPosItem *currItem;
BlockNumber blkno;
@@ -924,7 +924,10 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
*/
if (scan->parallel_scan != NULL)
{
- status = _bt_parallel_seize(scan, &blkno, true);
+ bool status;
+ BlockNumber lastcurrblkno;
+
+ status = _bt_parallel_seize(scan, &blkno, &lastcurrblkno, true);
/*
* Initialize arrays (when _bt_parallel_seize didn't already set up
@@ -942,7 +945,14 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
}
else if (blkno != InvalidBlockNumber)
{
- if (!_bt_parallel_readpage(scan, blkno, dir))
+ Assert(!so->needPrimScan);
+
+ /*
+ * We anticipated starting another primitive scan, but some other
+ * worker bet us to it. Bypass _bt_readfirstpage by calling
+ * _bt_readnextpage directly.
+ */
+ if (!_bt_readnextpage(scan, blkno, lastcurrblkno, dir))
return false;
goto readcomplete;
}
@@ -1427,10 +1437,6 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
}
}
- PredicateLockPage(rel, BufferGetBlockNumber(buf), scan->xs_snapshot);
-
- _bt_initialize_more_data(so, dir);
-
/* position to the precise item on the page */
offnum = _bt_binsrch(rel, &inskey, buf);
Assert(!BTScanPosIsValid(so->currPos));
@@ -1455,21 +1461,8 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
* for the page. For example, when inskey is both < the leaf page's high
* key and > all of its non-pivot tuples, offnum will be "maxoff + 1".
*/
- if (!_bt_readpage(scan, dir, offnum, true))
- {
- /*
- * There's no actually-matching data on this page. Try to advance to
- * the next page. Return false if there's no matching data at all.
- */
- _bt_unlockbuf(scan->indexRelation, so->currPos.buf);
- if (!_bt_steppage(scan, dir))
- return false;
- }
- else
- {
- /* We have at least one item to return as scan's first item */
- _bt_drop_lock_and_maybe_pin(scan, &so->currPos);
- }
+ if (!_bt_readfirstpage(scan, offnum, dir))
+ return false;
readcomplete:
/* OK, itemIndex says what to return */
@@ -1545,14 +1538,6 @@ _bt_next(IndexScanDesc scan, ScanDirection dir)
* that there can be no more matching tuples in the current scan direction
* (could just be for the current primitive index scan when scan has arrays).
*
- * _bt_first caller passes us an offnum returned by _bt_binsrch, which might
- * be an out of bounds offnum such as "maxoff + 1" in certain corner cases.
- * _bt_checkkeys will stop the scan as soon as an equality qual fails (when
- * its scan key was marked required), so _bt_first _must_ pass us an offnum
- * exactly at the beginning of where equal tuples are to be found. When we're
- * passed an offnum past the end of the page, we might still manage to stop
- * the scan on this page by calling _bt_checkkeys against the high key.
- *
* In the case of a parallel scan, caller must have called _bt_parallel_seize
* prior to calling this function; this function will invoke
* _bt_parallel_release before returning.
@@ -1563,6 +1548,7 @@ static bool
_bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
bool firstPage)
{
+ Relation rel = scan->indexRelation;
BTScanOpaque so = (BTScanOpaque) scan->opaque;
Page page;
BTPageOpaque opaque;
@@ -1582,18 +1568,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
page = BufferGetPage(so->currPos.buf);
opaque = BTPageGetOpaque(page);
- /* allow next page be processed by parallel worker */
- if (scan->parallel_scan)
- {
- if (ScanDirectionIsForward(dir))
- pstate.prev_scan_page = opaque->btpo_next;
- else
- pstate.prev_scan_page = BufferGetBlockNumber(so->currPos.buf);
-
- _bt_parallel_release(scan, pstate.prev_scan_page);
- }
-
- indnatts = IndexRelationGetNumberOfAttributes(scan->indexRelation);
+ indnatts = IndexRelationGetNumberOfAttributes(rel);
arrayKeys = so->numArrayKeys != 0;
minoff = P_FIRSTDATAKEY(opaque);
maxoff = PageGetMaxOffsetNumber(page);
@@ -1615,8 +1590,12 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
/*
* We note the buffer's block number so that we can release the pin later.
* This allows us to re-read the buffer if it is needed again for hinting.
+ * Also save the page's sibling links; this tells us where to step
+ * right/left to after we're done reading items from this page.
*/
so->currPos.currPage = BufferGetBlockNumber(so->currPos.buf);
+ so->currPos.prevPage = opaque->btpo_prev;
+ so->currPos.nextPage = opaque->btpo_next;
/*
* We save the LSN of the page as we read it, so that we know whether it
@@ -1625,13 +1604,6 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
*/
so->currPos.lsn = BufferGetLSNAtomic(so->currPos.buf);
- /*
- * we must save the page's right-link while scanning it; this tells us
- * where to step right to after we're done with these items. There is no
- * corresponding need for the left-link, since splits always go right.
- */
- so->currPos.nextPage = opaque->btpo_next;
-
/* initialize tuple workspace to empty */
so->currPos.nextTupleOffset = 0;
@@ -1640,6 +1612,20 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
* good.
*/
Assert(BTScanPosIsPinned(so->currPos));
+ Assert(!P_IGNORE(opaque));
+
+ /* allow next page to be processed by parallel worker */
+ if (scan->parallel_scan)
+ {
+ if (ScanDirectionIsForward(dir))
+ _bt_parallel_release(scan, so->currPos.nextPage,
+ so->currPos.currPage);
+ else
+ _bt_parallel_release(scan, so->currPos.prevPage,
+ so->currPos.currPage);
+ }
+
+ PredicateLockPage(rel, so->currPos.currPage, scan->xs_snapshot);
/*
* Prechecking the value of the continuescan flag for the last item on the
@@ -1804,7 +1790,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
IndexTuple itup = (IndexTuple) PageGetItem(page, iid);
int truncatt;
- truncatt = BTreeTupleGetNAtts(itup, scan->indexRelation);
+ truncatt = BTreeTupleGetNAtts(itup, rel);
pstate.prechecked = false; /* precheck didn't cover HIKEY */
_bt_checkkeys(scan, &pstate, arrayKeys, itup, truncatt);
}
@@ -1934,7 +1920,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
* We don't need to visit page to the left when no more matches will
* be found there
*/
- if (!pstate.continuescan || P_LEFTMOST(opaque))
+ if (!pstate.continuescan)
so->currPos.moreLeft = false;
Assert(itemIndex >= 0);
@@ -2035,20 +2021,21 @@ _bt_savepostingitem(BTScanOpaque so, int itemIndex, OffsetNumber offnum,
/*
* _bt_steppage() -- Step to next page containing valid data for scan
*
- * On entry, if so->currPos.buf is valid the buffer is pinned but not locked;
- * if pinned, we'll drop the pin before moving to next page. The buffer is
- * not locked on entry.
+ * On entry, if so->currPos.buf is valid the buffer is pinned but not locked.
+ * This is a wrapper on _bt_readnextpage that performs final steps for the
+ * current page (including dropping its pin). It sets up the _bt_readnextpage
+ * call using either local state saved by the last _bt_readpage call, or using
+ * shared parallel scan state (by seizing the parallel scan).
*
- * For success on a scan using a non-MVCC snapshot we hold a pin, but not a
- * read lock, on that page. If we do not hold the pin, we set so->currPos.buf
- * to InvalidBuffer. We return true to indicate success.
+ * Parallel scan callers that have already seized the scan should directly
+ * call _bt_readnextpage, rather than calling here.
*/
static bool
_bt_steppage(IndexScanDesc scan, ScanDirection dir)
{
BTScanOpaque so = (BTScanOpaque) scan->opaque;
- BlockNumber blkno = InvalidBlockNumber;
- bool status;
+ BlockNumber blkno,
+ lastcurrblkno;
Assert(BTScanPosIsValid(so->currPos));
@@ -2090,7 +2077,7 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
* In effect, btrestpos leaves advancing the arrays up to the first
* _bt_readpage call (that takes place after it has restored markPos).
*/
- Assert(so->markPos.dir == dir);
+ Assert(so->markDir == dir);
if (so->needPrimScan)
{
if (ScanDirectionIsForward(dir))
@@ -2098,93 +2085,193 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
else
so->markPos.moreLeft = true;
}
+
+ /* mark/restore not supported by parallel scans */
+ Assert(!scan->parallel_scan);
}
- if (ScanDirectionIsForward(dir))
+ /* release the previously pinned leaf page buffer, if any */
+ BTScanPosUnpinIfPinned(so->currPos);
+
+ /* Walk to the next page with data */
+ if (!scan->parallel_scan)
{
- /* Walk right to the next page with data */
- if (scan->parallel_scan != NULL)
- {
- /*
- * Seize the scan to get the next block number; if the scan has
- * ended already, bail out.
- */
- status = _bt_parallel_seize(scan, &blkno, false);
- if (!status)
- {
- /* release the previous buffer, if pinned */
- BTScanPosUnpinIfPinned(so->currPos);
- BTScanPosInvalidate(so->currPos);
- return false;
- }
- }
- else
- {
- /* Not parallel, so use the previously-saved nextPage link. */
+ /* Not parallel, so use nextPage/prevPage from local scan state */
+ if (ScanDirectionIsForward(dir))
blkno = so->currPos.nextPage;
- }
-
- /* Remember we left a page with data */
- so->currPos.moreLeft = true;
-
- /* release the previous buffer, if pinned */
- BTScanPosUnpinIfPinned(so->currPos);
+ else
+ blkno = so->currPos.prevPage;
+ lastcurrblkno = so->currPos.currPage;
}
else
{
- /* Remember we left a page with data */
- so->currPos.moreRight = true;
-
- if (scan->parallel_scan != NULL)
+ /*
+ * Seize the scan to get the nextPage and currPage from shared
+ * parallel state
+ */
+ if (!_bt_parallel_seize(scan, &blkno, &lastcurrblkno, false))
{
- /*
- * Seize the scan to get the current block number; if the scan has
- * ended already, bail out.
- */
- status = _bt_parallel_seize(scan, &blkno, false);
- BTScanPosUnpinIfPinned(so->currPos);
- if (!status)
- {
- BTScanPosInvalidate(so->currPos);
- return false;
- }
- }
- else
- {
- /* Not parallel, so just use our own notion of the current page */
- blkno = so->currPos.currPage;
+ /* don't call _bt_parallel_done */
+ return false;
}
}
- if (!_bt_readnextpage(scan, blkno, dir))
- return false;
+ /*
+ * _bt_readnextpage just works off of blkno/lastcurrblkno now
+ *
+ * (It does still expect so->currPos.moreLeft/so->currPos.moreLeft to
+ * remain however the last _bt_readpage call for lastcurrblkno set them.)
+ */
+ BTScanPosInvalidate(so->currPos);
- /* We have at least one item to return as scan's next item */
- _bt_drop_lock_and_maybe_pin(scan, &so->currPos);
+ if (!_bt_readnextpage(scan, blkno, lastcurrblkno, dir))
+ return false;
return true;
}
/*
- * _bt_readnextpage() -- Read next page containing valid data for scan
+ * _bt_readfirstpage() -- Read first page containing valid data for _bt_first
+ *
+ * _bt_first caller passes us an offnum returned by _bt_binsrch, which might
+ * be an out of bounds offnum such as "maxoff + 1" in certain corner cases.
+ * _bt_checkkeys will stop the scan as soon as an equality qual fails (when
+ * its scan key was marked required), so _bt_first _must_ pass us an offnum
+ * exactly at the beginning of where equal tuples are to be found. When we're
+ * passed an offnum past the end of the page, we might still manage to stop
+ * the scan on this page by calling _bt_checkkeys against the high key. See
+ * _bt_readpage for full details.
+ *
+ * On entry, so->currPos.buf must be pinned and locked. Also, parallel scan
+ * callers must have seized the scan before calling here.
*
* On success exit, so->currPos is updated to contain data from the next
- * interesting page, and we return true. Caller must release the lock (and
- * maybe the pin) on the buffer on success exit.
+ * interesting page, and we return true. We hold a pin on the buffer on
+ * success exit, except for scans that _bt_drop_lock_and_maybe_pin indicates
+ * can safely have their pin dropped to avoid blocking progress by VACUUM.
*
- * If there are no more matching records in the given direction, we drop all
- * locks and pins, set so->currPos.buf to InvalidBuffer, and return false.
+ * If there are no more matching records in the given direction, we set
+ * so->currPos.buf to InvalidBuffer, and return false. Note that this could
+ * just signal the end of the current primitive index scan.
+ *
+ * We always release the scan for a parallel scan caller, regardless of
+ * success or failure; we do this by calling _bt_parallel_release at the
+ * earliest opportunity.
*/
static bool
-_bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
+_bt_readfirstpage(IndexScanDesc scan, OffsetNumber offnum, ScanDirection dir)
{
BTScanOpaque so = (BTScanOpaque) scan->opaque;
- Relation rel;
+
+ Assert(BufferIsValid(so->currPos.buf));
+
+ /* Initialize currPos for so->currPos.buf */
+ if (so->needPrimScan)
+ {
+ Assert(so->numArrayKeys);
+
+ so->currPos.moreLeft = true;
+ so->currPos.moreRight = true;
+ so->needPrimScan = false;
+ }
+ else if (ScanDirectionIsForward(dir))
+ {
+ so->currPos.moreLeft = false;
+ so->currPos.moreRight = true;
+ }
+ else
+ {
+ so->currPos.moreLeft = true;
+ so->currPos.moreRight = false;
+ }
+ so->markDir = dir;
+ so->numKilled = 0; /* just paranoia */
+ so->markItemIndex = -1; /* ditto */
+
+ if (_bt_readpage(scan, dir, offnum, true))
+ {
+ /*
+ * _bt_readpage succeeded. Drop the lock (and maybe the pin) on
+ * so->currPos.buf in preparation for btgettuple returning tuples.
+ */
+ Assert(BTScanPosIsPinned(so->currPos));
+ _bt_drop_lock_and_maybe_pin(scan, &so->currPos);
+ return true;
+ }
+
+ /* There's no actually-matching data on the page in so->currPos.buf */
+ _bt_unlockbuf(scan->indexRelation, so->currPos.buf);
+
+ /* Call _bt_readnextpage through its _bt_steppage wrapper function */
+ if (!_bt_steppage(scan, dir))
+ return false;
+
+ /*
+ * _bt_readpage for a later page succeeded. There's no need to call
+ * _bt_drop_lock_and_maybe_pin here, though; _bt_readnextpage already did
+ * that for us.
+ */
+ return true;
+}
+
+/*
+ * _bt_readnextpage() -- Read next page containing valid data for _bt_next
+ *
+ * Caller's blkno is the next interesting page's link, taken from either the
+ * previously-saved right link or left link. lastcurrblkno is the page that
+ * was current at the point where the blkno link was saved, which we use to
+ * reason about concurrent page splits/page deletions during backwards scans
+ * (_bt_parallel_seize also requires it, regardless of scan direction).
+ *
+ * We work off of blkno and lastcurrblkno. On entry, so->currPos must be
+ * invalid; caller shouldn't hold any locks or pins on any page. Parallel
+ * scan callers must have seized the scan before calling here (and must pass
+ * us the blkno + lastcurrblkno they obtained as part of seizing the scan).
+ * However, we still expect so->currPos.moreLeft/so->currPos.moreLeft to
+ * remain however the last _bt_readpage call (for lastcurrblkno) set them.
+ *
+ * On success exit, so->currPos is updated to contain data from the next
+ * interesting page, and we return true. We hold a pin on the buffer on
+ * success exit, except for scans that _bt_drop_lock_and_maybe_pin indicates
+ * can safely have their pin dropped to avoid blocking progress by VACUUM.
+ *
+ * If there are no more matching records in the given direction, we leave
+ * so->currPos.buf set to InvalidBuffer, and return false. No locks or pins
+ * are retained. Note that this could just signal the end of the current
+ * primitive index scan (in which case another call to _bt_first will be
+ * required to continue the scan in the current scan direction).
+ *
+ * On exit, we'll always have released the seized scan for parallel scan
+ * callers (regardless of whether we were successful or not).
+ */
+static bool
+_bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
+ BlockNumber lastcurrblkno, ScanDirection dir)
+{
+ Relation rel = scan->indexRelation;
+ BTScanOpaque so = (BTScanOpaque) scan->opaque;
Page page;
BTPageOpaque opaque;
- bool status;
- rel = scan->indexRelation;
+ Assert(!BTScanPosIsValid(so->currPos));
+
+ /*
+ * Initialize currPos for blkno.
+ *
+ * We know that the scan just read a page (lastcurrblkno), so remember
+ * that we left a place with potentially matching tuples to blkno's left
+ * (or to blkno's right, when we're scanning backwards).
+ *
+ * Don't reset moreRight (or moreLeft if this is a backwards scan) here.
+ * If there is no more data to the right of lastcurrblkno, then there
+ * certainly cannot be any more to the right of its blkno right sibling
+ * (when we're scanning backwards then moreLeft similarly applies to both
+ * lastcurrblkno and blkno).
+ */
+ if (ScanDirectionIsForward(dir))
+ so->currPos.moreLeft = true;
+ else
+ so->currPos.moreRight = true;
if (ScanDirectionIsForward(dir))
{
@@ -2197,7 +2284,6 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
if (blkno == P_NONE || !so->currPos.moreRight)
{
_bt_parallel_done(scan);
- BTScanPosInvalidate(so->currPos);
return false;
}
/* check for interrupts while we're not holding any buffer lock */
@@ -2209,7 +2295,6 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
/* check for deleted page */
if (!P_IGNORE(opaque))
{
- PredicateLockPage(rel, blkno, scan->xs_snapshot);
/* see if there are any matches on this page */
/* note that this will clear moreRight if we can stop */
if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque), false))
@@ -2217,86 +2302,54 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
}
else if (scan->parallel_scan != NULL)
{
- /* allow next page be processed by parallel worker */
- _bt_parallel_release(scan, opaque->btpo_next);
+ /* _bt_readpage not called, so do this for ourselves */
+ _bt_parallel_release(scan, opaque->btpo_next, blkno);
}
- /* nope, keep going */
if (scan->parallel_scan != NULL)
{
+ /* no matching tuples, so seize a page to the right */
_bt_relbuf(rel, so->currPos.buf);
- status = _bt_parallel_seize(scan, &blkno, false);
- if (!status)
+
+ if (!_bt_parallel_seize(scan, &blkno, &lastcurrblkno, false))
{
- BTScanPosInvalidate(so->currPos);
+ /* don't call _bt_parallel_done */
return false;
}
}
else
{
+ /* no matching tuples, so consider following right link */
+ lastcurrblkno = blkno;
blkno = opaque->btpo_next;
_bt_relbuf(rel, so->currPos.buf);
}
+
+ BTScanPosInvalidate(so->currPos); /* working off new blkno now */
}
}
else
{
- /*
- * Should only happen in parallel cases, when some other backend
- * advanced the scan.
- */
- if (so->currPos.currPage != blkno)
- {
- BTScanPosUnpinIfPinned(so->currPos);
- so->currPos.currPage = blkno;
- }
-
- /* Done if we know that the left sibling link isn't of interest */
- if (!so->currPos.moreLeft)
- {
- BTScanPosUnpinIfPinned(so->currPos);
- _bt_parallel_done(scan);
- BTScanPosInvalidate(so->currPos);
- return false;
- }
-
- /*
- * Walk left to the next page with data. This is much more complex
- * than the walk-right case because of the possibility that the page
- * to our left splits while we are in flight to it, plus the
- * possibility that the page we were on gets deleted after we leave
- * it. See nbtree/README for details.
- *
- * It might be possible to rearrange this code to have less overhead
- * in pinning and locking, but that would require capturing the left
- * sibling block number when the page is initially read, and then
- * optimistically starting there (rather than pinning the page twice).
- * It is not clear that this would be worth the complexity.
- */
- if (BTScanPosIsPinned(so->currPos))
- _bt_lockbuf(rel, so->currPos.buf, BT_READ);
- else
- so->currPos.buf = _bt_getbuf(rel, so->currPos.currPage, BT_READ);
-
for (;;)
{
- /* Done if we know that the left sibling link isn't of interest */
- if (!so->currPos.moreLeft)
+ /*
+ * if we're at end of scan, give up and mark parallel scan as
+ * done, so that all the workers can finish their scan
+ */
+ if (blkno == P_NONE || !so->currPos.moreLeft)
{
- _bt_relbuf(rel, so->currPos.buf);
_bt_parallel_done(scan);
- BTScanPosInvalidate(so->currPos);
return false;
}
- /* Step to next physical page */
- so->currPos.buf = _bt_walk_left(rel, so->currPos.buf);
+ /* move left at least one page (checks for interrupts, too) */
+ so->currPos.buf = _bt_lock_and_validate_left(rel, &blkno,
+ lastcurrblkno);
- /* if we're physically at end of index, return failure */
if (so->currPos.buf == InvalidBuffer)
{
+ /* must have been a concurrent deletion of leftmost page */
_bt_parallel_done(scan);
- BTScanPosInvalidate(so->currPos);
return false;
}
@@ -2309,7 +2362,6 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
opaque = BTPageGetOpaque(page);
if (!P_IGNORE(opaque))
{
- PredicateLockPage(rel, BufferGetBlockNumber(so->currPos.buf), scan->xs_snapshot);
/* see if there are any matches on this page */
/* note that this will clear moreLeft if we can stop */
if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page), false))
@@ -2317,100 +2369,80 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
}
else if (scan->parallel_scan != NULL)
{
- /* allow next page be processed by parallel worker */
- _bt_parallel_release(scan, BufferGetBlockNumber(so->currPos.buf));
+ /* _bt_readpage not called, so do this for ourselves */
+ Assert(P_ISHALFDEAD(opaque));
+ _bt_parallel_release(scan, opaque->btpo_prev, blkno);
}
- /*
- * For parallel scans, get the last page scanned as it is quite
- * possible that by the time we try to seize the scan, some other
- * worker has already advanced the scan to a different page. We
- * must continue based on the latest page scanned by any worker.
- */
if (scan->parallel_scan != NULL)
{
+ /* no matching tuples, so seize a page to the left */
_bt_relbuf(rel, so->currPos.buf);
- status = _bt_parallel_seize(scan, &blkno, false);
- if (!status)
+
+ if (!_bt_parallel_seize(scan, &blkno, &lastcurrblkno, false))
{
- BTScanPosInvalidate(so->currPos);
+ /* don't call _bt_parallel_done */
return false;
}
- so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
}
+ else
+ {
+ /* no matching tuples, so consider following left link */
+ lastcurrblkno = blkno;
+ blkno = opaque->btpo_prev;
+ _bt_relbuf(rel, so->currPos.buf);
+ }
+
+ BTScanPosInvalidate(so->currPos); /* working off new blkno now */
}
}
- return true;
-}
-
-/*
- * _bt_parallel_readpage() -- Read current page containing valid data for scan
- *
- * On success, release lock and maybe pin on buffer. We return true to
- * indicate success.
- */
-static bool
-_bt_parallel_readpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
-{
- BTScanOpaque so = (BTScanOpaque) scan->opaque;
-
- Assert(!so->needPrimScan);
-
- _bt_initialize_more_data(so, dir);
-
- if (!_bt_readnextpage(scan, blkno, dir))
- return false;
-
- /* We have at least one item to return as scan's next item */
+ /*
+ * _bt_readpage succeeded. Drop the lock (and maybe the pin) on
+ * so->currPos.buf in preparation for btgettuple returning tuples.
+ */
+ Assert(BTScanPosIsPinned(so->currPos));
_bt_drop_lock_and_maybe_pin(scan, &so->currPos);
return true;
}
/*
- * _bt_walk_left() -- step left one page, if possible
+ * _bt_lock_and_validate_left() -- lock caller's left sibling blkno,
+ * recovering from concurrent page splits/page deletions when necessary
*
- * The given buffer must be pinned and read-locked. This will be dropped
- * before stepping left. On return, we have pin and read lock on the
- * returned page, instead.
+ * Called during backwards scans, to deal with their unique concurrency rules.
*
- * Returns InvalidBuffer if there is no page to the left (no lock is held
- * in that case).
+ * blkno points to the block number of the page that we expect to move the
+ * scan to. We'll successfully move the scan there when we find that its
+ * right sibling link points to lastcurrblkno, the page that we just finished
+ * reading. Otherwise, we have to figure out which page is the correct one to
+ * visit next the hard way, which involves reasoning about both concurrent
+ * page splits and concurrent page deletions. See nbtree/README for details.
+ *
+ * On return, we have both a pin and a read lock on the returned page, whose
+ * block number will be set in *blkno. Returns InvalidBuffer if there is no
+ * page to the left (no lock is held in that case).
*
* It is possible for the returned leaf page to be half-dead; caller must
* check that condition and step left again when required.
*/
static Buffer
-_bt_walk_left(Relation rel, Buffer buf)
+_bt_lock_and_validate_left(Relation rel, BlockNumber *blkno,
+ BlockNumber lastcurrblkno)
{
- Page page;
- BTPageOpaque opaque;
-
- page = BufferGetPage(buf);
- opaque = BTPageGetOpaque(page);
+ BlockNumber origblkno = *blkno; /* detects circular links */
for (;;)
{
- BlockNumber obknum;
- BlockNumber lblkno;
- BlockNumber blkno;
+ Buffer buf;
+ Page page;
+ BTPageOpaque opaque;
int tries;
- /* if we're at end of tree, release buf and return failure */
- if (P_LEFTMOST(opaque))
- {
- _bt_relbuf(rel, buf);
- break;
- }
- /* remember original page we are stepping left from */
- obknum = BufferGetBlockNumber(buf);
- /* step left */
- blkno = lblkno = opaque->btpo_prev;
- _bt_relbuf(rel, buf);
/* check for interrupts while we're not holding any buffer lock */
CHECK_FOR_INTERRUPTS();
- buf = _bt_getbuf(rel, blkno, BT_READ);
+ buf = _bt_getbuf(rel, *blkno, BT_READ);
page = BufferGetPage(buf);
opaque = BTPageGetOpaque(page);
@@ -2427,21 +2459,26 @@ _bt_walk_left(Relation rel, Buffer buf)
tries = 0;
for (;;)
{
- if (!P_ISDELETED(opaque) && opaque->btpo_next == obknum)
+ if (!P_ISDELETED(opaque) && opaque->btpo_next == lastcurrblkno)
{
/* Found desired page, return it */
return buf;
}
if (P_RIGHTMOST(opaque) || ++tries > 4)
break;
- blkno = opaque->btpo_next;
- buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
+ /* step right */
+ *blkno = opaque->btpo_next;
+ buf = _bt_relandgetbuf(rel, buf, *blkno, BT_READ);
page = BufferGetPage(buf);
opaque = BTPageGetOpaque(page);
}
- /* Return to the original page to see what's up */
- buf = _bt_relandgetbuf(rel, buf, obknum, BT_READ);
+ /*
+ * Return to the original page (usually the page most recently read by
+ * _bt_readpage, which is passed by caller as lastcurrblkno) to see
+ * what's up with sibling links
+ */
+ buf = _bt_relandgetbuf(rel, buf, lastcurrblkno, BT_READ);
page = BufferGetPage(buf);
opaque = BTPageGetOpaque(page);
if (P_ISDELETED(opaque))
@@ -2457,8 +2494,8 @@ _bt_walk_left(Relation rel, Buffer buf)
if (P_RIGHTMOST(opaque))
elog(ERROR, "fell off the end of index \"%s\"",
RelationGetRelationName(rel));
- blkno = opaque->btpo_next;
- buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
+ *blkno = opaque->btpo_next;
+ buf = _bt_relandgetbuf(rel, buf, *blkno, BT_READ);
page = BufferGetPage(buf);
opaque = BTPageGetOpaque(page);
if (!P_ISDELETED(opaque))
@@ -2466,9 +2503,10 @@ _bt_walk_left(Relation rel, Buffer buf)
}
/*
- * Now return to top of loop, resetting obknum to point to this
- * nondeleted page, and try again.
+ * remember that this page is actually now the leftmost one whose
+ * key space we've already read all tuples from
*/
+ lastcurrblkno = BufferGetBlockNumber(buf);
}
else
{
@@ -2477,11 +2515,22 @@ _bt_walk_left(Relation rel, Buffer buf)
* to the left got split or deleted. Without this check, we'd go
* into an infinite loop if there's anything wrong.
*/
- if (opaque->btpo_prev == lblkno)
+ if (opaque->btpo_prev == origblkno)
elog(ERROR, "could not find left sibling of block %u in index \"%s\"",
- obknum, RelationGetRelationName(rel));
- /* Okay to try again with new lblkno value */
+ lastcurrblkno, RelationGetRelationName(rel));
+ /* Okay to try again, since left sibling link changed */
}
+
+ if (P_LEFTMOST(opaque))
+ {
+ /* previous leftmost page must have been concurrently deleted */
+ _bt_relbuf(rel, buf);
+ break;
+ }
+
+ /* next loop iteration will start over with a clean slate */
+ *blkno = origblkno = opaque->btpo_prev;
+ _bt_relbuf(rel, buf);
}
return InvalidBuffer;
@@ -2605,7 +2654,6 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
return false;
}
- PredicateLockPage(rel, BufferGetBlockNumber(buf), scan->xs_snapshot);
page = BufferGetPage(buf);
opaque = BTPageGetOpaque(page);
Assert(P_ISLEAF(opaque));
@@ -2632,26 +2680,11 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
/* remember which buffer we have pinned */
so->currPos.buf = buf;
- _bt_initialize_more_data(so, dir);
-
/*
* Now load data from the first page of the scan.
*/
- if (!_bt_readpage(scan, dir, start, true))
- {
- /*
- * There's no actually-matching data on this page. Try to advance to
- * the next page. Return false if there's no matching data at all.
- */
- _bt_unlockbuf(scan->indexRelation, so->currPos.buf);
- if (!_bt_steppage(scan, dir))
- return false;
- }
- else
- {
- /* We have at least one item to return as scan's first item */
- _bt_drop_lock_and_maybe_pin(scan, &so->currPos);
- }
+ if (!_bt_readfirstpage(scan, start, dir))
+ return false;
/* OK, itemIndex says what to return */
currItem = &so->currPos.items[so->currPos.itemIndex];
@@ -2661,33 +2694,3 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
return true;
}
-
-/*
- * _bt_initialize_more_data() -- initialize moreLeft, moreRight and scan dir
- * from currPos
- */
-static inline void
-_bt_initialize_more_data(BTScanOpaque so, ScanDirection dir)
-{
- so->currPos.dir = dir;
- if (so->needPrimScan)
- {
- Assert(so->numArrayKeys);
-
- so->currPos.moreLeft = true;
- so->currPos.moreRight = true;
- so->needPrimScan = false;
- }
- else if (ScanDirectionIsForward(dir))
- {
- so->currPos.moreLeft = false;
- so->currPos.moreRight = true;
- }
- else
- {
- so->currPos.moreLeft = true;
- so->currPos.moreRight = false;
- }
- so->numKilled = 0; /* just paranoia */
- so->markItemIndex = -1; /* ditto */
-}
diff --git a/src/backend/access/nbtree/nbtutils.c b/src/backend/access/nbtree/nbtutils.c
index b4ba51357..03d5c671f 100644
--- a/src/backend/access/nbtree/nbtutils.c
+++ b/src/backend/access/nbtree/nbtutils.c
@@ -2442,7 +2442,7 @@ new_prim_scan:
so->needPrimScan = true; /* ...but call _bt_first again */
if (scan->parallel_scan)
- _bt_parallel_primscan_schedule(scan, pstate->prev_scan_page);
+ _bt_parallel_primscan_schedule(scan, so->currPos.currPage);
/* Caller's tuple doesn't match the new qual */
return false;
--
2.45.2
