Hi, Peter. On Fri, Sep 29, 2023 at 4:57 AM Peter Geoghegan <p...@bowt.ie> wrote: > On Fri, Sep 22, 2023 at 7:24 AM Alexander Korotkov <aekorot...@gmail.com> > wrote: > > The thing is that NULLs could appear in the middle of matching values. > > > > # WITH t (a, b) AS (VALUES ('a', 'b'), ('a', NULL), ('b', 'a')) > > SELECT a, b, (a, b) > ('a', 'a') FROM t ORDER BY (a, b); > > a | b | ?column? > > ---+------+---------- > > a | b | t > > a | NULL | NULL > > b | a | t > > (3 rows) > > > > So we can't just skip the row comparison operator, because we can meet > > NULL at any place. > > But why would SK_ROW_HEADER be any different? Is it related to this > existing case inside _bt_check_rowcompare()?: > > if (subkey->sk_flags & SK_ISNULL) > { > /* > * Unlike the simple-scankey case, this isn't a disallowed case. > * But it can never match. If all the earlier row comparison > * columns are required for the scan direction, we can stop the > * scan, because there can't be another tuple that will succeed. > */ > if (subkey != (ScanKey) DatumGetPointer(skey->sk_argument)) > subkey--; > if ((subkey->sk_flags & SK_BT_REQFWD) && > ScanDirectionIsForward(dir)) > *continuescan = false; > else if ((subkey->sk_flags & SK_BT_REQBKWD) && > ScanDirectionIsBackward(dir)) > *continuescan = false; > return false; > }
Yes, exactly. Our row comparison operators don't match if there is any null inside the row. But you can find these rows within the matching range. > I noticed that you're not initializing so->firstPage correctly for the > _bt_endpoint() path, which is used when the initial position of the > scan is either the leftmost or rightmost page. That is, it's possible > to reach _bt_readpage() without having reached the point in > _bt_first() where you initialize so->firstPage to "true". Good catch, thank you! > It would probably make sense if the flag was initialized to "false" in > the same way as most other scan state is already, somewhere in > nbtree.c. Probably in btrescan(). Makes sense, initialisation is added. ------ Regards, Alexander Korotkov
0001-Skip-checking-of-scan-keys-required-for-direction-v8.patch
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