Re: [PATCH] btree_gist: fix union implementation for variable length columns

On Wednesday, July 11, 2018 7:26:40 PM CEST Tom Lane wrote:
> Pavel Raiskup <praiskup@redhat.com> writes:
> > On Monday, July 9, 2018 7:41:59 PM CEST Tom Lane wrote:
> >> Hi Pavel!  For patches that purport to resolve bugs, we usually like to
> >> add a regression test case that demonstrates the bug in unpatched code.
> >> Can you provide a small test case that does so?  (The BZ you pointed to
> >> doesn't seem to address this...)
> 
> > Turns out the problem is only related to bit/bit varying type (so the
> > patch comments need to be reworded properly, at least) since those are the
> > only types which have implemented the f_l2n() callback.
> 
> What I'm failing to wrap my head around is why this code exists at all.
> AFAICS, gbt_bit_xfrm just forces the bitstring to be zero-padded out to
> an INTALIGN boundary, which it wouldn't necessarily be otherwise.
> But why bother?  That should have no effect on the behavior of bit_cmp().

The gbt_bit_xfrm() method actually also skips the varbit header (number of
bits, stored in VarBit.bit_len), the magic is done by VARBITS macro.  If
the header is dropped, it's OK to just use existing byteacmp() for bit
array comparison.

Pavel