Sort is not avoided even when the has an index on a the column being ordered,
for a query with id != -1 predicate.
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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;
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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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