CREATE TABLE TBL (
    FLD  INTEGER,
    ID   INTEGER
);

CREATE INDEX TBL_IDX_FLD ON TBL (FLD);
CREATE INDEX TBL_IDX_ID ON TBL (ID);

set term ^;
execute block
AS
  declare cnt integer;
  declare i integer;
  declare k integer;
begin
  i = 0;
  k = 0;
  cnt = 0;
  while (cnt < 100000) do
  begin
    insert into tbl(id, fld) values(:i, :k);
    i = i + 1;
    if (i > 5) then i = 0;
    k = k + 1;
    if (k > 4663) then k = 0;
    cnt = cnt + 1;
  end
  execute statement 'set statistics index TBL_IDX_FLD';
  execute statement 'set statistics index TBL_IDX_ID';
end^
commit^

select count(*) from tbl a where
    exists (select * from tbl b where b.id = 0 and a.fld = b.fld)

select count(*) from tbl a where
    exists (select * from tbl b where b.id+0 = 0 and a.fld = b.fld)

select i.rdb$index_name, cast(i.rdb$statistics as varchar(100)) from rdb$indices i WHERE i.rdb$relation_name = 'TBL'

TBL (550)
    Primary pointer page: 1254, Index root page: 1264
    Total formats: 1, used formats: 1
    Average record length: 11.50, total records: 100000
    Average version length: 0.00, total versions: 0, max versions: 0
    Average fragment length: 0.00, total fragments: 0, max fragments: 0
    Average unpacked length: 12.00, compression ratio: 1.04
    Pointer pages: 1, data page slots: 656
    Data pages: 656, average fill: 53%
    Primary pages: 656, secondary pages: 0, swept pages: 0
    Empty pages: 2, full pages: 653
    Fill distribution:
         0 - 19% = 2
        20 - 39% = 1
        40 - 59% = 653
        60 - 79% = 0
        80 - 99% = 0

    Index TBL_IDX_FLD (0)
        Root page: 1440, depth: 2, leaf buckets: 51, nodes: 100000
        Average node length: 4.03, total dup: 95336, max dup: 21
        Average key length: 2.05, compression ratio: 1.45
        Average prefix length: 2.92, average data length: 0.05
        Clustering factor: 100000, ratio: 1.00
        Fill distribution:
             0 - 19% = 1
            20 - 39% = 0
            40 - 59% = 0
            60 - 79% = 0
            80 - 99% = 50

    Index TBL_IDX_ID (1)
        Root page: 81493, depth: 2, leaf buckets: 50, nodes: 100000
        Average node length: 3.99, total dup: 99994, max dup: 16666
        Average key length: 2.00, compression ratio: 0.83
        Average prefix length: 1.67, average data length: 0.00
        Clustering factor: 3924, ratio: 0.04
        Fill distribution:
             0 - 19% = 0
            20 - 39% = 1
            40 - 59% = 0
            60 - 79% = 0
            80 - 99% = 49

TBL (484)
    Primary pointer page: 62957, Index root page: 62958
    Average record length: 11.50, total records: 100000
    Average version length: 0.00, total versions: 0, max versions: 0
    Data pages: 654, data page slots: 654, average fill: 53%
    Fill distribution:
         0 - 19% = 0
        20 - 39% = 1
        40 - 59% = 653
        60 - 79% = 0
        80 - 99% = 0

    Index TBL_IDX_FLD (0)
        Depth: 2, leaf buckets: 123, nodes: 100000
        Average data length: 0.00, total dup: 95336, max dup: 21
        Fill distribution:
             0 - 19% = 1
            20 - 39% = 0
            40 - 59% = 68
            60 - 79% = 48
            80 - 99% = 6

    Index TBL_IDX_ID (1)
        Depth: 2, leaf buckets: 144, nodes: 100000
        Average data length: 0.00, total dup: 99994, max dup: 16666
        Fill distribution:
             0 - 19% = 0
            20 - 39% = 0
            40 - 59% = 142
            60 - 79% = 0
            80 - 99% = 2

select id, count(*) from tbl group by 1

ID	COUNT
0	16667
1	16667
2	16667
3	16667
4	16666
5	16666