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From "Suresh Thalamati (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-3367) Sort is not avoided even when the has an index on a the column being ordered, for a query with id != -1 predicate.
Date Wed, 30 Jan 2008 20:27:34 GMT
Sort is not avoided even when the has an index on a the column being ordered,  for a query
with id != -1 predicate.
-------------------------------------------------------------------------------------------------------------------

                 Key: DERBY-3367
                 URL: https://issues.apache.org/jira/browse/DERBY-3367
             Project: Derby
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 10.3.2.1
            Reporter: Suresh Thalamati
         Attachments: derby.log

Sort is not avoided even when the has an index on a the column being ordered, 

Repro:

go.ddl:
-------

connect 'jdbc:derby:testdb;create=true';

create table t1 (i int, j int, vc varchar(30));
insert into t1 values (1, -1, 'minus one');
insert into t1 values (2, 2, 'two'), (3, 3, 'trois'), (3, -3, 'minus three'), (4, 4, 'four');

insert into t1 select * from t1 where j > 0;
insert into t1 select * from t1 where j > 0;
insert into t1 select * from t1 where j > 0;
insert into t1 select * from t1 where j > 0;
insert into t1 select * from t1 where j > 0;
insert into t1 select * from t1 where j > 0;
insert into t1 select * from t1 where j > 0;
insert into t1 select * from t1 where j > 0;
insert into t1 select * from t1 where j > 0;
insert into t1 select * from t1 where j > 0;

create index ix on t1 (j);
disconnect all;

exit;

go.sql:
-------

connect 'jdbc:derby:testdb';

get cursor c1 as 'select j, vc from t1 order by j asc';
next c1;
close c1;

get cursor c1 as 'select j, vc from t1 where j != -1 order by j asc';
next c1;
close c1;



--

After running "go.sql", if you look at the derby.log file you'll see that the 
query with no predicate does an index scan and only has to read 1 row from disk 
before the cursor is closed.  But the query _with_ a predicate does a table
scan an has to read 3074 rows from disk, and sort them, just to return 
the first one in the result set. 

In the repro, it looks fast. But If the data is large, 
which was the case in my  application.  
The table was: 
create table t2 (i int, j int, vc varchar(15000)); 
and loaded with 13000 rows. It takes almost minute to get the first row ,
for the query "select j, vc from t1 where j != -1 order by j asc'"












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