Submitted by: @pavel-zotov

Attachments:
firebird.20200109_102731.704191.WI-T4_0_0_1714.stack_trace.7z

Consider following script:

recreate table test0(n decfloat);
commit;

insert into test0 values( 0 );
insert into test0 values( 0 );
commit;

set sqlda_display on;
select n as test0_n from test0;
set sqlda_display off;
commit;

set list on;
set explain on;

select n as n_grouped_from_test0 from test0 group by 1; --- [ 1 ]

select distinct n as n_uniq_from_test0 from test0; -- [ 2 ]

select count(distinct n) as count_uniq_from_test0 from test0; -- [ 3 ]

Statement [ 3 ] will raise crash on recent FB 4.x.

firebird.log will contain after this:

IMAGE-PC1 Thu Jan 9 10:27:31 2020
Access violation.
The code attempted to access a virtual
address without privilege to do so.
This exception will cause the Firebird server
to terminate abnormally.

IMAGE-PC1 Thu Jan 9 10:27:31 2020
INET/inet_error: read errno = 10054, server host = Image-PC1, address = fe80::fcf1:e33c:e924:969d%16/3439

IMAGE-PC1 Thu Jan 9 10:27:31 2020
REMOTE INTERFACE/gds__detach: Unsuccesful detach from database.
Uncommitted work may have been lost.
Error writing data to the connection.
send_packet/send

Stack trace can be seen in attached .7z

PS.

If replace records with unique values:

insert into test0 values( 0 );
insert into test0 values( 1 );

-- then count(distinct N) works OK.

Checked on WI-T4.0.0.1714

Commits: 272a49e 473afd3