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https://issues.apache.org/jira/browse/DERBY-6858?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15125570#comment-15125570
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somebody commented on DERBY-6858:
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Those are interesting findings. I'm glad we could at least rule out the
deferrable constraints, though it remains to be seen if that makes the problem
easier to solve.
Sounds like there might need to be some optimization done to disk access and
caching pages in memory. Though maybe that's not the issue.
Perhaps it's the format of the derby disk files or how they are read. Perhaps
the derby disk files are read too many times for such queries if the page size
is small. Perhaps they are read in smaller chunks when a better strategy is to
read larger blocks at once and cache them in memory.
Perhaps the multiple reads (if that is what's happening) are from different
access paths. For example, perhaps the same data is loaded multiple time. For
example, once for the select, once for the update, and once for updating each
index. If this is the case, it would be better to cache the data for use when
doing later operations, or aggregate these operations together somehow (if that
is possible).
What do you think Bryan? Do you have a theory?
> 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, repro.java, 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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