select * from t order by s

C:\MIX\firebird\OLTPTEST>isql -q
SQL> create database '192.168.0.220/3333:/var/db/fb30/tmp201405071909.fdb'; commit;
SQL> show version;
ISQL Version: WI-V2.5.3.26730 Firebird 2.5
Server version:
Firebird/linux AMD64 (access method), version "LI-T3.0.0.31113 Firebird 3.0 Alpha 2"
Firebird/linux AMD64 (remote server), version "LI-T3.0.0.31113 Firebird 3.0 Alpha 2/tcp (oel64)/P12"
Firebird/x86/Windows NT (remote interface), version "WI-V2.5.3.26730 Firebird 2.5/tcp (csprog)/P12"
on disk structure version 12.0
SQL> create collation coll_ns for utf8 from unicode 'NUMERIC-SORT=1'; commit;
SQL> recreate table t1( s varchar(36) character set utf8 collate coll_ns ); commit;
SQL> insert into t1 select ''||row_number()over() from rdb$types, rdb$types; commit;
SQL> create index t1_s on t1(s);  -- ::: NB :::  NON- unique index
SQL> commit;
SQL> set planonly;
SQL> select * from t1 order by s;

PLAN (T1 ORDER T1_S) -- Ok

SQL>
SQL> set planonly;
SQL> drop index t1_s; commit;

SQL> create  UNIQUE  index t1_s_ unq  on t1(s); commit;
SQL> set planonly;
SQL> select * from t1 order by s;

PLAN SORT (T1 NATURAL) -- Почему не "PLAN (T1 ORDER T1_S_UNQ)" ?

SQL> set planonly;

SQL> select * from t1 where s>'';

PLAN (T1 INDEX (T1_S_UNQ))