Avoid pointer chasing in _bt_readpage inner loop.

Make _bt_readpage pass down the current scan direction to various
utility functions within its pstate variable.  Also have _bt_readpage
work off of a local copy of scan->ignore_killed_tuples within its
per-tuple loop (rather than using scan->ignore_killed_tuples directly).

Testing has shown that this significantly benefits large range scans,
which are naturally able to take full advantage of the pstate.startikey
optimization added by commit 8a510275.  Running a pgbench script with a
"SELECT abalance FROM pgbench_accounts WHERE aid BETWEEN ..." query
shows an increase in transaction throughput of over 5%.  There also
appears to be a small performance benefit when running pgbench's
built-in select-only script.

Follow-up to commit 65d6acbc.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Victor Yegorov <vyegorov@gmail.com>
Discussion: https://postgr.es/m/CAH2-WzmwMwcwKFgaf+mYPwiz3iL4AqpXnwtW_O0vqpWPXRom9Q@mail.gmail.com

diff --git a/src/backend/access/nbtree/nbtreadpage.c b/src/backend/access/nbtree/nbtreadpage.c
index 00df52b81bed1a0a217a01463a1a76f69f145025..ac67b6f7a34127f4ef47d9771424d21ec918cb0b 100644 (file)
--- a/src/backend/access/nbtree/nbtreadpage.c
+++ b/src/backend/access/nbtree/nbtreadpage.c
@@ -31,6 +31,7 @@
 typedef struct BTReadPageState
 {
    /* Input parameters, set by _bt_readpage for _bt_checkkeys */
+   ScanDirection dir;          /* current scan direction */
    OffsetNumber minoff;        /* Lowest non-pivot tuple's offset */
    OffsetNumber maxoff;        /* Highest non-pivot tuple's offset */
    IndexTuple  finaltup;       /* Needed by scans with array keys */
@@ -140,7 +141,8 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
    OffsetNumber minoff;
    OffsetNumber maxoff;
    BTReadPageState pstate;
-   bool        arrayKeys;
+   bool        arrayKeys,
+               ignore_killed_tuples = scan->ignore_killed_tuples;
    int         itemIndex,
                indnatts;
 
@@ -151,7 +153,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
    so->currPos.prevPage = opaque->btpo_prev;
    so->currPos.nextPage = opaque->btpo_next;
    /* delay setting so->currPos.lsn until _bt_drop_lock_and_maybe_pin */
-   so->currPos.dir = dir;
+   pstate.dir = so->currPos.dir = dir;
    so->currPos.nextTupleOffset = 0;
 
    /* either moreRight or moreLeft should be set now (may be unset later) */
@@ -244,7 +246,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
             * If the scan specifies not to return killed tuples, then we
             * treat a killed tuple as not passing the qual
             */
-           if (scan->ignore_killed_tuples && ItemIdIsDead(iid))
+           if (ignore_killed_tuples && ItemIdIsDead(iid))
            {
                offnum = OffsetNumberNext(offnum);
                continue;
@@ -402,7 +404,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
             * uselessly advancing to the page to the left.  This is similar
             * to the high key optimization used by forward scans.
             */
-           if (scan->ignore_killed_tuples && ItemIdIsDead(iid))
+           if (ignore_killed_tuples && ItemIdIsDead(iid))
            {
                if (offnum > minoff)
                {
@@ -1157,8 +1159,8 @@ _bt_checkkeys(IndexScanDesc scan, BTReadPageState *pstate, bool arrayKeys,
              IndexTuple tuple, int tupnatts)
 {
    TupleDesc   tupdesc = RelationGetDescr(scan->indexRelation);
-   BTScanOpaque so = (BTScanOpaque) scan->opaque;
-   ScanDirection dir = so->currPos.dir;
+   BTScanOpaque so PG_USED_FOR_ASSERTS_ONLY = (BTScanOpaque) scan->opaque;
+   ScanDirection dir = pstate->dir;
    int         ikey = pstate->startikey;
    bool        res;
 
@@ -2059,8 +2061,7 @@ static void
 _bt_checkkeys_look_ahead(IndexScanDesc scan, BTReadPageState *pstate,
                         int tupnatts, TupleDesc tupdesc)
 {
-   BTScanOpaque so = (BTScanOpaque) scan->opaque;
-   ScanDirection dir = so->currPos.dir;
+   ScanDirection dir = pstate->dir;
    OffsetNumber aheadoffnum;
    IndexTuple  ahead;
 
@@ -2193,7 +2194,7 @@ _bt_advance_array_keys(IndexScanDesc scan, BTReadPageState *pstate,
 {
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    Relation    rel = scan->indexRelation;
-   ScanDirection dir = so->currPos.dir;
+   ScanDirection dir = pstate ? pstate->dir : ForwardScanDirection;
    int         arrayidx = 0;
    bool        beyond_end_advance = false,
                skip_array_advanced = false,