Optimize creation of expression and partial indices

[dyemanov]

The idea is to share an expression/condition request between multiple processed records in cases when it's possible (index creation, garbage collection, validation).

Test case:

recreate table t (id int) ;
insert into t select row_number() over() from rdb$types, rdb$types, rdb$types;
commit;

set stat on;

create index it0 on t (id);
create index it1 on t (id) where id is not null;
create index it2 on t computed (id+0);
create index it3 on t computed (id+0) where id is not null;

-- v5

SQL> create index it0 on t (id);
Elapsed time = 9.331 sec

SQL> create index it1 on t (id) where id is not null;
Elapsed time = 69.126 sec

SQL> create index it2 on t computed (id+0);
Elapsed time = 72.234 sec

SQL> create index it3 on t computed (id+0) where id is not null;
Elapsed time = 129.571 sec

-- v5 with patch

SQL> create index it0 on t (id);
Elapsed time = 9.369 sec

SQL> create index it1 on t (id) where id is not null;
Elapsed time = 10.920 sec

SQL> create index it2 on t computed (id+0);
Elapsed time = 12.464 sec

SQL> create index it3 on t computed (id+0) where id is not null;
Elapsed time = 14.466 sec

[hvlad]

Before optimization, condition\expression's request was unwound for every record processed, now it is done after processing of a many records, is it safe ?
Note, condition\expression could be complex and contain cursors, for example.