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https://issues.apache.org/jira/browse/DERBY-6858?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15117027#comment-15117027
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somebody commented on DERBY-6858:
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Hi Bryan,
Thanks for looking into this. I will take a look at your test program.
As for you question, what I stated, 1 row in ParentUpdate and 2500 rows in
ChildUpdate are correct. The update statement you have questions about (see
you post) is for ChildUpdate, which I mentioned has 2500 records. Remember
parentName is a column in ChildUpdate (it references ParentUpdate.name). Ie.
ChildUpdate.parentName -> ParentUpdate.name and the update statement is setting
ChildUpdate.parentName.
Perhaps your confusion is coming from the fact that you expect there to be a
constraint violation after updating ChildUpdate (which is completely
reasonable). Looking at my post I realize I never put the update of
ParentUpdate statement into the description. I'm not sure how I missed this.
Sorry about that.
To provide more details, the following are the two create table statements
(executed in the order they are listed below):
CREATE TABLE ParentUpdate(name varchar(255),value int NOT NULL, CONSTRAINT
ParentUpdate_PK primary key (name))
CREATE TABLE ChildUpdate(parentName varchar(255),name varchar(255),value
int,data varchar(1000),primary key (name), CONSTRAINT
ChildUpdate_TO_ParentUpdate FOREIGN KEY (parentName) REFERENCES
ParentUpdate(name) DEFERRABLE INITIALLY DEFERRED)
the following two sql statements are being executed in the same transaction on
1 record in ParentUpadte and 2500 records in ChildUpdate under both the fast
and slow scenario with the only difference being the extra byte in the data
column of ChildUpdate causing drastically different behaviour:
update ChildUpdate set parentName = 'Parent 2' where parentName = 'Parent 1'
update ParentUpdate set name = 'Parent 2' where name = 'Parent 1'
So to answer you question, there is only one record in ParentUpdate and the
reason is that the foreign key ChildUpdate_TO_ParentUpdate is deferred and both
of the above update statements are run in the same transaction.
I will look at your test program but in the mean time it would be great if you
can also try to debug the issue so we get to the bottom of it faster.
If you have any more questions please let me know and I will provide all the
information I can. Thanks again for looking at this.
> Apache Derby simple update statement performance becomes 1500% worse when
> adding one byte to a column
> -----------------------------------------------------------------------------------------------------
>
> Key: DERBY-6858
> URL: https://issues.apache.org/jira/browse/DERBY-6858
> Project: Derby
> Issue Type: Bug
> Affects Versions: 10.11.1.1, 10.12.1.1
> Environment: windows 7 64 bit
> Reporter: somebody
> Priority: Blocker
> Attachments: repro.java
>
>
> I have 2 tables as follows:
> ParentUpdate
> name varchar(255)
> value int not null
> primary key: name
> ChildUpdate
> parentName varchar(255)
> name varchar(255)
> value int
> data varchar(1000)
> primary key: name foreign key: parentName to ParentUpdate.name
> When I run the statement "update ChildUpdate set parentName = 'Parent 2'
> where parentName = 'Parent 1'" with 2500 records in the ChildUpdate table and
> 1 record in the ParentUpdate table with only a single byte difference in data
> size in the ChildUpdate table, the performance decreases by 15 times.
> When the ChildUpdate data column has exactly 14 bytes of the same character
> the runtime of the above query is about 500 milliseconds. When I add one more
> byte to the data column of ChildUpdate the performance all of a sudden
> becomes about 7500 milliseconds.
> If i then decrease the data size back to 14 from 15 it's fast again. When i
> put it back to 15 it's slow again. This is reproducible every time.
> Can you please help me figure out how to get the same fast performance
> without such seemingly random behaviour.
> The query plans are below for both cases.
> projection = true
> constructor time (milliseconds) = 0
> open time (milliseconds) = 0
> next time (milliseconds) = 16
> close time (milliseconds) = 16
> restriction time (milliseconds) = 0
> projection time (milliseconds) = 0
> optimizer estimated row count: 51.50
> optimizer estimated cost: 796.12
> Source result set:
> Table Scan ResultSet for CHILDUPDATE at read committed isolation
> level using exclusive row locking chosen by the optimizer
> Number of opens = 1
> Rows seen = 2500
> Rows filtered = 0
> Fetch Size = 1
> constructor time (milliseconds) = 0
> open time (milliseconds) = 15
> next time (milliseconds) = 16
> close time (milliseconds) = 16
> next time in milliseconds/row = 0
> scan information:
> Bit set of columns fetched={0, 1}
> Number of columns fetched=2
> Number of pages visited=41
> Number of rows qualified=2500
> Number of rows visited=2500
> 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
> optimizer estimated row count: 51.50
> optimizer estimated cost: 796.12
> total time: ~500 milliseconds
> and the slow version
> Statement Name:
> null
> Statement Text:
> update ChildUpdate set parentName = 'Parent 2' where parentName = 'Parent
> 1'
> Parse Time: 0
> Bind Time: 0
> Optimize Time: 0
> Generate Time: 0
> Compile Time: 0
> Execute Time: -1453199485700
> Begin Compilation Timestamp : 2016-01-19 05:31:25.684
> End Compilation Timestamp : 2016-01-19 05:31:25.684
> Begin Execution Timestamp : 2016-01-19 05:31:25.7
> End Execution Timestamp : 2016-01-19 05:31:33.141
> Statement Execution Plan Text:
> Update ResultSet using row locking:
> deferred: true
> Rows updated = 2500
> Indexes updated = 2
> Execute Time = -1453199485747
> Normalize ResultSet:
> Number of opens = 1
> Rows seen = 2500
> constructor time (milliseconds) = 0
> open time (milliseconds) = 0
> next time (milliseconds) = 47
> close time (milliseconds) = 0
> optimizer estimated row count: 51.50
> optimizer estimated cost: 810.94
> Source result set:
> Project-Restrict ResultSet (3):
> Number of opens = 1
> Rows seen = 2500
> Rows filtered = 0
> restriction = false
> projection = true
> constructor time (milliseconds) = 0
> open time (milliseconds) = 0
> next time (milliseconds) = 32
> close time (milliseconds) = 0
> restriction time (milliseconds) = 0
> projection time (milliseconds) = 0
> optimizer estimated row count: 51.50
> optimizer estimated cost: 810.94
> Source result set:
> Project-Restrict ResultSet (2):
> Number of opens = 1
> Rows seen = 2500
> Rows filtered = 0
> restriction = false
> projection = true
> constructor time (milliseconds) = 0
> open time (milliseconds) = 0
> next time (milliseconds) = 32
> close time (milliseconds) = 0
> restriction time (milliseconds) = 0
> projection time (milliseconds) = 0
> optimizer estimated row count: 51.50
> optimizer estimated cost: 810.94
> Source result set:
> Index Scan ResultSet for CHILDUPDATE using index TESTINDEX at
> read committed isolation level using exclusive row locking chosen by the
> optimizer
> Number of opens = 1
> Rows seen = 2500
> Rows filtered = 0
> Fetch Size = 1
> constructor time (milliseconds) = 0
> open time (milliseconds) = 0
> next time (milliseconds) = 32
> close time (milliseconds) = 0
> next time in milliseconds/row = 0
> scan information:
> Bit set of columns fetched={0, 1, 2}
> Number of columns fetched=3
> Number of deleted rows visited=0
> Number of pages visited=42
> Number of rows qualified=2500
> Number of rows visited=2500
> Scan type=btree
> Tree height=2
> start position:
> None
> stop position:
> None
> qualifiers:
> Column[0][0] Id: 1
> Operator: =
> Ordered nulls: false
> Unknown return value: false
> Negate comparison result: false
> optimizer estimated row count: 51.50
> optimizer estimated cost: 810.94
> total time: ~7 seconds 500 milliseconds
> please also see post:
> http://stackoverflow.com/questions/34874762/apache-derby-simple-update-statement-performance-becomes-1500-worse-when-adding
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