Submitted by: @dyemanov

Test case:

select 1 from rdb$database where timestamp '01.01.2021 13:00:00 +03:00' = timestamp '01.01.2021 12:00:00 +02:00';
-- Values are surely equal:

CONSTANT 

============
1

select 1 from (select timestamp '01.01.2021 13:00:00 +03:00' as ts from rdb$database) natural join (select timestamp '01.01.2021 12:00:00 +02:00' as ts from rdb$database);
-- OOPS! No record is returned!

select 1 from rdb$database where cast(10 as decfloat) = cast(10.000 as decfloat);
-- Values are surely equal:

CONSTANT 

============
1

select 1 from (select cast(10 as decfloat) as df from rdb$database) natural join (select cast(10.000 as decfloat) as df from rdb$database);
-- OOPS! No record is returned!

select 1 from rdb$database where cast('+0' as decfloat) = cast('-0' as decfloat);
-- Values are surely equal:

CONSTANT 

============
1

select 1 from (select cast('+0' as decfloat) as df from rdb$database) natural join (select cast('-0' as decfloat) as df from rdb$database);
-- OOPS! No record is returned!

And also another case with approximate zeros (similar to CORE3547):

select 1 from rdb$database where cast('+0' as float) = cast('-0' as float);
-- Values are surely equal:

CONSTANT 

============
1

select 1 from (select cast('+0' as float) as f from rdb$database) natural join (select cast('-0' as float) as f from rdb$database);
-- OOPS! No record is returned!

select 1 from rdb$database where cast('+0' as double precision) = cast('-0' as double precision);
-- Values are surely equal:

CONSTANT 

============
1

select 1 from (select cast('+0' as double precision) as d from rdb$database) natural join (select cast('-0' as double precision) as d from rdb$database);
-- OOPS! No record is returned!

Commits: b6ef39d