Re: Index range search optimization

On Fri, Sep 22, 2023 at 7:24 AM Alexander Korotkov <aekorotkov@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;
        }

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".

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().

--
Peter Geoghegan