CLONE - Query optimizer should not make poor choices when optimizing IN and
WHERE clauses
-----------------------------------------------------------------------------------------
Key: DERBY-713
URL: http://issues.apache.org/jira/browse/DERBY-713
Project: Derby
Type: Improvement
Components: SQL
Versions: 10.0.2.0
Environment: all
Reporter: Daniel James Neades
Consider a simple case of -
A table tbl has 10000 rows, there is a primary key index on i1
and the query in question is
select * from tbl where i1 in (-1,100000)
derby does a table scan of the entire table even though the "IN" list has only
two values and the comparison is on a field that has an index.
Briefly looking at the code, it seems like we insert a between and use the IN
list to get the start and stop values for the scan. Thus the range of the
values in the "IN" list here plays an important role.
Thus if the query was changed to select * from tbl where i1 in (-1, 1), an
index scan would be chosen.
It would be nice if we could do something clever in this case where there is
clearly an index on the field and the number of values in the IN list is known.
Maybe use the rowcount estimate and the IN list size to do some optimizations.
- consider the length of the "IN" list to do searches on the table. ie use the
IN list values to do index key searches on the table,
-or try to convert it to a join. Use the "IN" list values to create a temporary
table and do a join. It is most likely that the optimizer will choose the table
with "IN" list here as the outer table in the join and thus will do key
searches on the larger table.
-------------------------------------------------------------------
some query plans that I logged using derby.language.logQueryPlan=true for some
similar queries:
Table has ascending values from 0 - 9999 for i1. primary key index on i1.
GMT Thread[UT0,5,main] (XID = 19941), (SESSIONID = 0), select * from scanfixed
where i1 in (-1,9999,9998,9997,9996,9995,9994,9993,9992,9991,9990) *******
Project-Restrict ResultSet (2):
Number of opens = 1
Rows seen = 10000
Rows filtered = 9990
restriction = true
projection = false
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
restriction time (milliseconds) = 0
projection time (milliseconds) = 0
optimizer estimated row count: 750.38
optimizer estimated cost: 8579.46
Source result set:
Table Scan ResultSet for SCANFIXED at read committed isolation level
using instantaneous share row locking chosen by the optimizer
Number of opens = 1
Rows seen = 10000
Rows filtered = 0
Fetch Size = 16
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
next time in milliseconds/row = 0
scan information:
Bit set of columns fetched=All
Number of columns fetched=9
Number of pages visited=417
Number of rows qualified=10000
Number of rows visited=10000
Scan type=heap
start position:
null stop position:
null qualifiers:
Column[0][0] Id: 0
Operator: <=
Ordered nulls: false
Unknown return value: false
Negate comparison result: false
Column[0][1] Id: 0
Operator: <
Ordered nulls: false
Unknown return value: true
Negate comparison result: true
optimizer estimated row count: 750.38
optimizer estimated cost: 8579.46
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
l
2004-10-14 18:59:47.577 GMT Thread[UT0,5,main] (XID = 19216), (SESSIONID = 0),
select * from scanfixed where i1 in
(9999,9998,9997,9996,9995,9994,9993,9992,9991,9990) ******* Project-Restrict
ResultSet (3):
Number of opens = 1
Rows seen = 10
Rows filtered = 0
restriction = true
projection = true
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
restriction time (milliseconds) = 0
projection time (milliseconds) = 0
optimizer estimated row count: 4.80
optimizer estimated cost: 39.53
Source result set:
Index Row to Base Row ResultSet for SCANFIXED:
Number of opens = 1
Rows seen = 10
Columns accessed from heap = {0, 1, 2, 3, 4, 5, 6, 7, 8}
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
optimizer estimated row count: 4.80
optimizer estimated cost: 39.53
Index Scan ResultSet for SCANFIXED using index SCANFIXEDX at
read committed isolation level using instantaneous share row locking chosen by
the optimizer
Number of opens = 1
Rows seen = 10
Rows filtered = 0
Fetch Size = 16
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
next time in milliseconds/row = 0
scan information:
Bit set of columns fetched=All
Number of columns fetched=2
Number of deleted rows visited=0
Number of pages visited=2
Number of rows qualified=10
Number of rows visited=10
Scan type=btree
Tree height=2
start position:
>= on first 1 column(s).
Ordered null semantics on the following columns:
stop position:
> on first 1 column(s).
Ordered null semantics on the following columns:
qualifiers:
None
optimizer estimated row count: 4.80
optimizer estimated cost: 39.53
--
This message is automatically generated by JIRA.
-
If you think it was sent incorrectly contact one of the administrators:
http://issues.apache.org/jira/secure/Administrators.jspa
-
For more information on JIRA, see:
http://www.atlassian.com/software/jira
