On 07/04/2021 09:00, Heikki Linnakangas wrote:
On 08/03/2021 19:06, Andrey Borodin wrote:
There were numerous GiST-build-related patches in this thread. Yet uncommitted
is a patch with sortsupport routines for btree_gist contrib module.
Here's its version which needs review.
Reviewing this now again. One thing caught my eye:
+static int
+gbt_bit_sort_build_cmp(Datum a, Datum b, SortSupport ssup)
+{
+ return DatumGetInt32(DirectFunctionCall2(byteacmp,
+
PointerGetDatum(a),
+
PointerGetDatum(b)));
+}
That doesn't quite match the sort order used by the comparison
functions, gbt_bitlt and such. The comparison functions compare the bits
first, and use the length as a tie-breaker. Using byteacmp() will
compare the "bit length" first. However, gbt_bitcmp() also uses
byteacmp(), so I'm a bit confused. So, huh?
Ok, I think I understand that now. In btree_gist, the *_cmp() function
operates on non-leaf values, and *_lt(), *_gt() et al operate on leaf
values. For all other datatypes, the leaf and non-leaf representation is
the same, but for bit/varbit, the non-leaf representation is different.
The leaf representation is VarBit, and non-leaf is just the bits without
the 'bit_len' field. That's why it is indeed correct for gbt_bitcmp() to
just use byteacmp(), whereas gbt_bitlt() et al compares the 'bit_len'
field separately. That's subtle, and 100% uncommented.
What that means for this patch is that gbt_bit_sort_build_cmp() should
*not* call byteacmp(), but bitcmp(). Because it operates on the original
datatype stored in the table.
- Heikki
