Unexpected error after deleting an index

[Noremos]

Here is a simple example. Create the follow metadata:

create table DOC(PAR_ID integer, EXC_ID integer, exe_name varchar(1000));
insert into DOC values(0, 1,'123r332');
commit;
CREATE INDEX DOC_INDEX ON DOC COMPUTED BY (upper(exe_name));
exit;

Then in a new connection:

Database: test.fdb, User: SYSDBA
SQL> set autoddl off;
SQL> drop index DOC_INDEX;
SQL> select * from DOC where EXC_ID = 1;
Statement failed, SQLSTATE = 42S12
there is no index in table DOC with id 0

The problem is that is kinda inconsistent. For example, if i call some select * from doc earlier, the error disappears

Database: test.fdb, User: SYSDBA
SQL> select * from DOC where PAR_ID = 1;
SQL> set autoddl off;
SQL> drop index DOC_INDEX;
SQL> select * from DOC where EXC_ID = 1;

PAR_ID                          0
EXC_ID                          1
EXE_NAME                        123r332

But this case is nothing compared to the next one. Same 3 calls, but now with two connections:

->start connect 1
->start connect 2
connect 1:
Database: localhost:test.fdb, User: SYSDBA
set autoddl off;
drop index DOC_INDEX;
select * from DOC where EXC_ID = 1;
The error...

connect 2: 
Database: localhost:test.fdb, User: SYSDBA
SQL> insert into DOC values(5, 7,'1,h7j332');
Statement failed, SQLSTATE = 42S12
Error during savepoint backout - transaction invalidated
-there is no index in table DOC with id 0
SQL> 

This is highly unexpected for regular users
Tested on the master branch (ffd31e3)

[dyemanov]

Regular users are commonly suggested to avoid (1) intermixing DDL and DML in a single transaction and (2) running DDL in concurrent environment. That said, I would still be good to have this issue resolved if possible.

[hvlad]

Why expression index is used in example ? Is it important ?

[dyemanov]

Yes, it's important (either expression or partial index is required).

[Noremos]

Why expression index is used in example ? Is it important ?

The error message comes from here

firebird/src/jrd/btr.cpp

Lines 968 to 994 in 78f2097

	if (idx->idx_flags & idx_expression)
	{
	MET_lookup_index_expression(tdbb, relation, idx);
	if (!idx->idx_expression)
	{
	if (tdbb->tdbb_flags & TDBB_sweeper)
	return false;
	// Definition of index expression is not found for index @1
	error = isc_idx_expr_not_found;
	}
	}
	if (!error && idx->idx_flags & idx_condition)
	{
	MET_lookup_index_condition(tdbb, relation, idx);
	if (!idx->idx_condition)
	{
	if (tdbb->tdbb_flags & TDBB_sweeper)
	return false;
	// Definition of index condition is not found for index @1
	error = isc_idx_cond_not_found;
	}
	}