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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-2641) Convert lang/staleplans.sql to JUnit
Date Tue, 15 May 2007 20:52:16 GMT

     [ https://issues.apache.org/jira/browse/DERBY-2641?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Mike Matrigali updated DERBY-2641:
----------------------------------


The intent in derby is definitely to try to pick index scan over table scan for tables with
small numbers of rows - EVEN if the performance might be better doing the table scan.  The
actual use problems were:
1) when you pick heap vs. index scan depending on isolation level often one had to set a TABLE
lock rather then individual row locks,
     this is a particular problem with serializable isolation.
2) more subtle is that if the index scan eliminates rows then we lock less rows than the heap
case where minimally we will row lock
     every row of the heap, this is especially problem with repeatable read and serializable.
Especially in case where derby needed to pick table locking to do correct isolation level
customers were seeing applications fail
on derby which would run otherwise unchanged.  Also this behavior met what most customers
expected.  They created an index,
the query used the index.  

This is a case where we choose to weigh concurrency over execution cost in the cost based
optimizer.  The hope is:
1) usually small number of row table performance does not matter.
2) often the small number of rows is actually growing but we happen to compile at app startup.

> Convert lang/staleplans.sql to JUnit
> ------------------------------------
>
>                 Key: DERBY-2641
>                 URL: https://issues.apache.org/jira/browse/DERBY-2641
>             Project: Derby
>          Issue Type: Improvement
>          Components: Test
>    Affects Versions: 10.3.0.0
>            Reporter: Knut Anders Hatlen
>         Assigned To: Knut Anders Hatlen
>            Priority: Minor
>         Attachments: derby-2641-1.diff, derby-2641-1.stat
>
>


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