RUNTIMESTATISTICS also provides information about the statement execution plan. The statement execution plan shows how long each node took to evaluate, how many rows were retrieved, whether an index was used, and so on. If an index was used, it shows the start and stop positions for the matching index scan. Looking at the plan can help you determine whether to add an index or to rewrite the query.
A statement execution plan is composed of a tree of result set nodes. A result set node represents the evaluation of one portion of the statement; it returns rows to a calling (or parent) node and can receive rows from a child node. A node can have one or more children. Starting from the top, if a node has children, it requests rows from the children. Usually only the execution plans of DML statements (queries, inserts, updates, and deletes, not dictionary object creation) are composed of more than one node.
SELECT * FROM Countries
Statement Name:
null
Statement Text:
select * from countries
Parse Time: 20
Bind Time: 10
Optimize Time: 50
Generate Time: 20
Compile Time: 100
Execute Time: 10
Begin Compilation Timestamp : 2005-05-25 09:16:21.24
End Compilation Timestamp : 2005-05-25 09:16:21.34
Begin Execution Timestamp : 2005-05-25 09:16:21.35
End Execution Timestamp : 2005-05-25 09:16:21.4
Statement Execution Plan Text:
Table Scan ResultSet for COUNTRIES at read committed isolation
level using instntaneous share row
locking chosen by the optimizer
Number of opens = 1
Rows seen = 114
Rows filtered = 0
Fetch Size = 16
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 10
close time (milliseconds) = 0
next time in milliseconds/row = 0
scan information:
Bit set of columns fetched=All
Number of columns fetched=3
Number of pages visited=3
Number of rows qualified=114
Number of rows visited=114
Scan type=heap
start position:
null stop position:
null qualifiers:
None
optimizer estimated row count: 119.00
optimizer estimated cost: 69.35
SELECT Country FROM Countries WHERE Region = 'Central America'
Statement Name:
null
Statement Text:
SELECT Country FROM Countries WHERE Region = 'Central America'
Parse Time: 10
Bind Time: 0
Optimize Time: 370
Generate Time: 10
Compile Time: 390
Execute Time: 0
Begin Compilation Timestamp : 2005-05-25 09:20:41.274
End Compilation Timestamp : 2005-05-25 09:20:41.664
Begin Execution Timestamp : 2005-05-25 09:20:41.674
End Execution Timestamp : 2005-05-25 09:20:41.674
Statement Execution Plan Text:
Project-Restrict ResultSet (2):
Number of opens = 1
Rows seen = 6
Rows filtered = 0
restriction = false
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: 11.90
optimizer estimated cost: 69.35
Source result set:
Table Scan ResultSet for COUNTRIES at read committed isolation level
using instantaneous share row locking chosen by the optimizer
Number of opens = 1
Rows seen = 6
Rows filtered = 0
Fetch Size = 16
constructor time (milliseconds) = 0
open time (milliseconds) = 10
next time (milliseconds) = 0
close time (milliseconds) = 0
next time in milliseconds/row = 0
scan information:
Bit set of columns fetched={0, 2}
Number of columns fetched=2
Number of pages visited=3
Number of rows qualified=6
Number of rows visited=114
Scan type=heap
start position:
null stop position:
null qualifiers:
Column[0][0] Id: 2
Operator: =
Ordered nulls: false
Unknown return value: false
Negate comparison result: false
optimizer estimated row count: 11.90
optimizer estimated cost: 69.35
Other, more complex queries such as joins and unions have other types of result set nodes.
For inserts, updates, and deletes, rows flow out of the top, where they are inserted, updated, or deleted. For selects (queries), rows flow out of the top into a result set that is returned to the user.
The Derby Reference Manual shows the many possible ResultSet nodes that might appear in an execution plan.
In addition, read DML statements and performance, for more information about some of the ways in which Derby executes statements.