recreate sequence g_asc;
recreate sequence g_desc;
recreate table test_asc(id bigint primary key using ascending index test_asc_id);
recreate table test_dec(id bigint primary key using descending index test_dec_id);

set term ^; execute block as declare n int = 50000000; begin while(n>0) do insert into test_asc values(gen_id(g_asc,1)) returning :n-1 into n; end^ set term ;^
commit;

set term ^; execute block as declare n int = 50000000; begin while(n>0) do insert into test_dec values(gen_id(g_desc, -1)) returning :n-1 into n; end^ set term ;^
commit;

2016-02-05T15:15:39.4020 (2608:0x7fae326ef040) EXECUTE_STATEMENT_FINISH
        e30 (ATT_7, SYSDBA:NONE, NONE, TCPv4:192.168.0.220/36061)
        /opt/fb30ss/bin/isql:2631
                (TRA_11, CONCURRENCY | WAIT | READ_WRITE)

Statement 27:
-------------------------------------------------------------------------------
execute block as declare n int = 50000000; begin while(n>0) do insert into test_asc values(gen_id(g_asc,1)) returning :n-1 into n; end
0 records fetched
 795093 ms, 568 read(s), 530075 write(s), 401730629 fetch(es), 151488906 mark(s)

Table                             Natural     Index    Update    Insert    Delete   Backout     Purge   Expunge
***************************************************************************************************************
RDB$PAGES                                                           187
TEST_ASC                                                       50000000



2016-02-05T15:32:38.0340 (2608:0x7fae326ef040) EXECUTE_STATEMENT_FINISH
        e30 (ATT_7, SYSDBA:NONE, NONE, TCPv4:192.168.0.220/36061)
        /opt/fb30ss/bin/isql:2631
                (TRA_20, CONCURRENCY | WAIT | READ_WRITE)

Statement 96:
-------------------------------------------------------------------------------
execute block as declare n int = 50000000; begin while(n>0) do insert into test_dec values(gen_id(g_desc, -1)) returning :n-1 into n; end
0 records fetched
 796282 ms, 563 read(s), 530098 write(s), 401750452 fetch(es), 151488927 mark(s)

Table                             Natural     Index    Update    Insert    Delete   Backout     Purge   Expunge
***************************************************************************************************************
RDB$PAGES                                                           187
TEST_DEC                                                       50000000

2016-02-05T15:35:11.6000 (2608:0x7fae326ef040) EXECUTE_STATEMENT_FINISH
        e30 (ATT_7, SYSDBA:NONE, NONE, TCPv4:192.168.0.220/36061)
        /opt/fb30ss/bin/isql:2631
                (TRA_27, CONCURRENCY | WAIT | READ_WRITE)

Statement 97:
-------------------------------------------------------------------------------
select * from test_asc where id between 25000000 and 26000000 order by id
1000001 records fetched
   3050 ms, 7640 read(s), 3001503 fetch(es)

Table                             Natural     Index    Update    Insert    Delete   Backout     Purge   Expunge
***************************************************************************************************************
TEST_ASC                                    1000001

 == vs  == 

Statement 403:
-------------------------------------------------------------------------------
select * from test_dec where id between -26000000 and -25000000 order by id desc
1000001 records fetched
   3088 ms, 7640 read(s), 3001503 fetch(es)

Table                             Natural     Index    Update    Insert    Delete   Backout     Purge   Expunge
***************************************************************************************************************
TEST_DEC                                    1000001

Database "/var/db/fb30/e30.fdb"
Database header page information:
	Flags			0
	Generation		46
	System Change Number	0
	Page size		8192
	ODS version		12.0
	Oldest transaction	32
	Oldest active		33
	Oldest snapshot		33
	Next transaction	34
	Sequence number		0
	Next attachment ID	11
	Implementation		HW=AMD/Intel/x64 little-endian OS=Linux CC=gcc
	Shadow count		0
	Page buffers		0
	Next header page	0
	Database dialect	3
	Creation date		Feb 2, 2016 18:57:15
	Attributes		

    Variable header data:
	*END*


Database file sequence:
File /var/db/fb30/e30.fdb is the only file

Analyzing database pages ...
TEST_ASC (128)
    Primary pointer page: 181, Index root page: 182
    Total formats: 1, used formats: 1
    Average record length: 10.66, total records: 50000000
    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: 16.00, compression ratio: 1.50
    Pointer pages: 188, data page slots: 306752
    Data pages: 306752, average fill: 55%
    Primary pages: 306752, secondary pages: 0, swept pages: 0
    Empty pages: 3, full pages: 306748
    Fill distribution:
	 0 - 19% = 3
	20 - 39% = 1
	40 - 59% = 306748
	60 - 79% = 0
	80 - 99% = 0

    Index TEST_ASC_ID (0)
	Root page: 3026, depth: 3, leaf buckets: 73987, nodes: 50000000
	Average node length: 11.68, total dup: 0, max dup: 0
	Average key length: 8.14, compression ratio: 1.11
	Average prefix length: 3.86, average data length: 5.14
	Clustering factor: 306749, ratio: 0.01
	Fill distribution:
	     0 - 19% = 0
	    20 - 39% = 0
	    40 - 59% = 3120
	    60 - 79% = 0
	    80 - 99% = 70867

TEST_DEC (129)
    Primary pointer page: 381495, Index root page: 381520
    Total formats: 1, used formats: 1
    Average record length: 10.66, total records: 50000000
    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: 16.00, compression ratio: 1.50
    Pointer pages: 188, data page slots: 306752
    Data pages: 306752, average fill: 55%
    Primary pages: 306752, secondary pages: 0, swept pages: 0
    Empty pages: 3, full pages: 306748
    Fill distribution:
	 0 - 19% = 3
	20 - 39% = 1
	40 - 59% = 306748
	60 - 79% = 0
	80 - 99% = 0

    Index TEST_DEC_ID (0)
	Root page: 384209, depth: 3, leaf buckets: 73987, nodes: 50000000
	Average node length: 11.68, total dup: 0, max dup: 0
	Average key length: 8.14, compression ratio: 1.11
	Average prefix length: 3.86, average data length: 5.14
	Clustering factor: 306749, ratio: 0.01
	Fill distribution:
	     0 - 19% = 0
	    20 - 39% = 0
	    40 - 59% = 3120
	    60 - 79% = 0
	    80 - 99% = 70867

2016-02-05T16:47:40.1080 (2608:0x7fae326ef040) EXECUTE_STATEMENT_FINISH
        /var/db/fb30/e30.fdb (ATT_5, SYSDBA:NONE, NONE, TCPv4:192.168.0.220/36065)
        /opt/fb30ss/bin/isql:2934
                (TRA_4, CONCURRENCY | WAIT | READ_WRITE)

Statement 95:
-------------------------------------------------------------------------------
execute block as declare n int = 50000000; begin while(n>0) do insert into test_dec values(gen_id( g_asc, 1 )) returning :n-1 into n; end
0 records fetched
 869686 ms, 1129 read(s), 887981 write(s), 408344253 fetch(es), 151846887 mark(s)

Table                             Natural     Index    Update    Insert    Delete   Backout     Purge   Expunge
***************************************************************************************************************
RDB$PAGES                                                           187
TEST_DEC                                                       50000000

SQL> set explain on; select * from test_dec where id between 25000000 and 26000000 order by id desc;

Select Expression
    -> Filter
        -> Table "TEST_DEC" Access By ID
            -> Index "TEST_DEC_ID" Range Scan (lower bound: 1/1, upper bound: 1/1)

Statement 31:
-------------------------------------------------------------------------------
select * from test_dec where id between 25000000 and 26000000 order by id desc
1000001 records fetched
   3124 ms, 9130 read(s), 3002993 fetch(es)

Table                             Natural     Index    Update    Insert    Delete
*********************************************************************************
TEST_DEC                                    1000001

TEST_DEC (128)
    Primary pointer page: 181, Index root page: 182
    Total formats: 1, used formats: 1
    Average record length: 10.66, total records: 50000000
    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: 16.00, compression ratio: 1.50
    Pointer pages: 188, data page slots: 306752
    Data pages: 306752, average fill: 55%
    Primary pages: 306752, secondary pages: 0, swept pages: 0
    Empty pages: 3, full pages: 306748
    Fill distribution:
         0 - 19% = 3
        20 - 39% = 1
        40 - 59% = 306748
        60 - 79% = 0
        80 - 99% = 0

    Index TEST_DEC_ID (0)
        Root page: 396870,  depth: 4, leaf buckets: 144575 , nodes: 50000000
        Average node length: 11.69, total dup: 0, max dup: 0
        Average key length: 8.14, compression ratio: 1.11
        Average prefix length: 3.85, average data length: 5.15
        Clustering factor: 306749,  ratio: 0.01 
        Fill distribution:
             0 - 19% = 0
            20 - 39% = 0
            40 - 59% = 144574
            60 - 79% = 0
            80 - 99% = 1