nbtree: Minor sibling link traversal tweaks.

Tweak some code comments for clarity, and relocate some local variable
declarations to the scope where they're actually used.

Follow-up to recent commit 1bd4bc85.

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 2275553be788ba3963f990aedf638045737a26a1..c33438c4bbd28ec69eff769388f18a7392b2c524 100644 (file)
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -1580,7 +1580,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
                                                                 so->currPos.currPage);
        }
 
-       /* initialize remaining currPos fields (before moreLeft/moreright) */
+       /* initialize remaining currPos fields (before moreLeft/moreRight) */
        so->currPos.lsn = BufferGetLSNAtomic(so->currPos.buf);
        so->currPos.dir = dir;
        so->currPos.nextTupleOffset = 0;
@@ -2154,7 +2154,7 @@ _bt_readfirstpage(IndexScanDesc scan, OffsetNumber offnum, ScanDirection dir)
        so->numKilled = 0;                      /* just paranoia */
        so->markItemIndex = -1;         /* ditto */
 
-       /* Initialize currPos for so->currPos */
+       /* Initialize so->currPos for the first page (page in so->currPos.buf) */
        if (so->needPrimScan)
        {
                Assert(so->numArrayKeys);
@@ -2175,7 +2175,7 @@ _bt_readfirstpage(IndexScanDesc scan, OffsetNumber offnum, ScanDirection dir)
        }
 
        /*
-        * Attempt to load matching tuples from the page in so->currPos.buf.
+        * Attempt to load matching tuples from the first page.
         *
         * Note that _bt_readpage will finish initializing the so->currPos fields.
         * _bt_readpage also releases parallel scan (even when it returns false).
@@ -2208,8 +2208,7 @@ _bt_readfirstpage(IndexScanDesc scan, OffsetNumber offnum, ScanDirection dir)
  * 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).
+ * reason about concurrent page splits/page deletions during backwards scans.
  *
  * On entry, caller shouldn't hold any locks or pins on any page (we work
  * directly off of blkno and lastcurrblkno instead).  Parallel scan callers
@@ -2234,8 +2233,6 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
 {
        Relation        rel = scan->indexRelation;
        BTScanOpaque so = (BTScanOpaque) scan->opaque;
-       Page            page;
-       BTPageOpaque opaque;
 
        Assert(so->currPos.currPage == lastcurrblkno || scan->parallel_scan != NULL);
        Assert(!BTScanPosIsPinned(so->currPos));
@@ -2251,14 +2248,14 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
 
        for (;;)
        {
-               /*
-                * if we're at end of scan, give up and mark parallel scan as done, so
-                * that all the workers can finish their scan
-                */
+               Page            page;
+               BTPageOpaque opaque;
+
                if (blkno == P_NONE ||
                        (ScanDirectionIsForward(dir) ?
                         !so->currPos.moreRight : !so->currPos.moreLeft))
                {
+                       /* most recent _bt_readpage call (for lastcurrblkno) ended scan */
                        _bt_parallel_done(scan);
                        BTScanPosInvalidate(so->currPos);
                        return false;