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https://issues.apache.org/jira/browse/DERBY-6858?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15156137#comment-15156137
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Mike Matrigali commented on DERBY-6858:
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note that deferred update is a different concept than deferred constraint
enforcement. on a per row updated cost it is always going to
cost more to do an update deferred rather than not.
Again in the most recent optimizer posts the slow case is when the optimizer
picks index scan, and thus must use deferred update. I believe the optimizer
is just
costing the "read" part of the query. In last case posted the estimate cost is:
index scan, deferred=true, "slow case":
optimizer estimated row count: 250.60
optimizer estimated cost: 844.70
actual rows qualified = all 2500
table scan, deferred=false, "fast case"
optimizer estimated row count: 958.00
optimizer estimated cost: 438.64
actual rows qualified = all 2500
What really would have been interesting would have been to have seen the costs
in the same run of the alternate query plans.
This use to be avaliable using some debug setting in debug server, not sure if
still available.
> 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: jvsisualvm-screenshot.jpg.png, profile.nps, 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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