Add nbtree amgettuple return item function.

This makes it easier to add precondition assertions.  We now assert that
the last call to _bt_readpage succeeded, and that the current item index
is within the bounds of the currPos items array.

Author: Peter Geoghegan <[email protected]>
Reviewed-By: Masahiro Ikeda <[email protected]>
Discussion: https://postgr.es/m/CAH2-WznFkEs9K1PtNruti5JjawY-dwj+gkaEh_k1ZE+1xLLGkA@mail.gmail.com

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 2786a8564f2e8a15a2707666727b006b076b5826..bf2dc9682ecdafc862f0d014d063dbad4638055b 100644 (file)
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -42,6 +42,7 @@ static int    _bt_setuppostingitems(BTScanOpaque so, int itemIndex,
 static inline void _bt_savepostingitem(BTScanOpaque so, int itemIndex,
                                       OffsetNumber offnum,
                                       ItemPointer heapTid, int tupleOffset);
+static inline void _bt_returnitem(IndexScanDesc scan, BTScanOpaque so);
 static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
 static bool _bt_readfirstpage(IndexScanDesc scan, OffsetNumber offnum,
                              ScanDirection dir);
@@ -867,8 +868,7 @@ _bt_compare(Relation rel,
  *     matching tuple(s) on the page has been loaded into so->currPos.  We'll
  *     drop all locks and hold onto a pin on page's buffer, except when
  *     _bt_drop_lock_and_maybe_pin dropped the pin to avoid blocking VACUUM.
- *     scan->xs_heaptid is set to the heap TID of the current tuple, and if
- *     requested, scan->xs_itup points to a copy of the index tuple.
+ *     _bt_returnitem sets the next item to return to scan on success exit.
  *
  * If there are no matching items in the index, we return false, with no
  * pins or locks held.  so->currPos will remain invalid.
@@ -890,7 +890,6 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
    ScanKeyData notnullkeys[INDEX_MAX_KEYS];
    int         keysz = 0;
    StrategyNumber strat_total;
-   BTScanPosItem *currItem;
 
    Assert(!BTScanPosIsValid(so->currPos));
 
@@ -950,7 +949,9 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
             */
            if (!_bt_readnextpage(scan, blkno, lastcurrblkno, dir, true))
                return false;
-           goto readcomplete;
+
+           _bt_returnitem(scan, so);
+           return true;
        }
    }
    else if (so->numArrayKeys && !so->needPrimScan)
@@ -1438,14 +1439,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
    if (!_bt_readfirstpage(scan, offnum, dir))
        return false;
 
-readcomplete:
-   /* OK, itemIndex says what to return */
-   Assert(BTScanPosIsValid(so->currPos));
-   currItem = &so->currPos.items[so->currPos.itemIndex];
-   scan->xs_heaptid = currItem->heapTid;
-   if (scan->xs_want_itup)
-       scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
-
+   _bt_returnitem(scan, so);
    return true;
 }
 
@@ -1456,9 +1450,8 @@ readcomplete:
  *     but is not locked, and so->currPos.itemIndex identifies which item was
  *     previously returned.
  *
- *     On successful exit, scan->xs_heaptid is set to the TID of the next
- *     heap tuple, and if requested, scan->xs_itup points to a copy of the
- *     index tuple.  so->currPos is updated as needed.
+ *     On success exit, so->currPos is updated as needed, and _bt_returnitem
+ *     sets the next item to return to the scan.  so->currPos remains valid.
  *
  *     On failure exit (no more tuples), we invalidate so->currPos.  It'll
  *     still be possible for the scan to return tuples by changing direction,
@@ -1468,7 +1461,6 @@ bool
 _bt_next(IndexScanDesc scan, ScanDirection dir)
 {
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
-   BTScanPosItem *currItem;
 
    Assert(BTScanPosIsValid(so->currPos));
 
@@ -1493,13 +1485,7 @@ _bt_next(IndexScanDesc scan, ScanDirection dir)
        }
    }
 
-   /* OK, itemIndex says what to return */
-   Assert(BTScanPosIsValid(so->currPos));
-   currItem = &so->currPos.items[so->currPos.itemIndex];
-   scan->xs_heaptid = currItem->heapTid;
-   if (scan->xs_want_itup)
-       scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
-
+   _bt_returnitem(scan, so);
    return true;
 }
 
@@ -1560,10 +1546,13 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
                                 so->currPos.currPage);
    }
 
-   /* initialize remaining currPos fields (before moreLeft/moreRight) */
+   /* initialize remaining currPos fields related to current page */
    so->currPos.lsn = BufferGetLSNAtomic(so->currPos.buf);
    so->currPos.dir = dir;
    so->currPos.nextTupleOffset = 0;
+   /* either moreLeft or moreRight should be set now (may be unset later) */
+   Assert(ScanDirectionIsForward(dir) ? so->currPos.moreRight :
+          so->currPos.moreLeft);
 
    PredicateLockPage(rel, so->currPos.currPage, scan->xs_snapshot);
 
@@ -2002,6 +1991,26 @@ _bt_savepostingitem(BTScanOpaque so, int itemIndex, OffsetNumber offnum,
        currItem->tupleOffset = tupleOffset;
 }
 
+/*
+ * Return the index item from so->currPos.items[so->currPos.itemIndex] to the
+ * index scan by setting the relevant fields in caller's index scan descriptor
+ */
+static inline void
+_bt_returnitem(IndexScanDesc scan, BTScanOpaque so)
+{
+   BTScanPosItem *currItem = &so->currPos.items[so->currPos.itemIndex];
+
+   /* Most recent _bt_readpage must have succeeded */
+   Assert(BTScanPosIsValid(so->currPos));
+   Assert(so->currPos.itemIndex >= so->currPos.firstItem);
+   Assert(so->currPos.itemIndex <= so->currPos.lastItem);
+
+   /* Return next item, per amgettuple contract */
+   scan->xs_heaptid = currItem->heapTid;
+   if (so->currTuples)
+       scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
+}
+
 /*
  * _bt_steppage() -- Step to next page containing valid data for scan
  *
@@ -2543,7 +2552,6 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
    Page        page;
    BTPageOpaque opaque;
    OffsetNumber start;
-   BTScanPosItem *currItem;
 
    Assert(!BTScanPosIsValid(so->currPos));
 
@@ -2593,12 +2601,6 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
    if (!_bt_readfirstpage(scan, start, dir))
        return false;
 
-   /* OK, itemIndex says what to return */
-   Assert(BTScanPosIsValid(so->currPos));
-   currItem = &so->currPos.items[so->currPos.itemIndex];
-   scan->xs_heaptid = currItem->heapTid;
-   if (scan->xs_want_itup)
-       scan->xs_itup = (IndexTuple) (so->currTuples + currItem->tupleOffset);
-
+   _bt_returnitem(scan, so);
    return true;
 }