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Reduce precedence writes [CORE2672] #3075

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firebird-issue-importer opened this issue Oct 9, 2009 · 6 comments
Closed

Reduce precedence writes [CORE2672] #3075

firebird-issue-importer opened this issue Oct 9, 2009 · 6 comments

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@firebird-issue-importer
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@firebird-issue-importer firebird-issue-importer commented Oct 9, 2009

Submitted by: @hvlad

Below is simplified description of how update is performed by the engine.

At first, update must read the record.
If record have backversion and backversion is ready to be garbage collected it must be erased.
Let primary page number is A and backversion page number is B.
Deletion of backversion creates dependency in precedence graph : A -> B, i.e. page A must be written after page B (as we just cleared backpointer at primary record version).
Then update must create and store new backversion. With high probablity it will be stored again on page B.
This create second dependency B -> A.
To prevent circle in precedence graph such case requires to write page A on disk clearing its dependencies.
This is so-called precedence write.
So, in the worst case we will write pages on disk for every updated record which make mass update performance terrible slow.

The likely issue happens if when engine performs update_in_place (for example second update of the same record in the same transaction).

Commits: 789b069

@firebird-issue-importer
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@firebird-issue-importer firebird-issue-importer commented Oct 9, 2009

Commented by: @hvlad

The offered solition is to search for page B in precedence graph before storing backversion and choose another page to not create circle in precedence graph and avoid precedence write.

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@firebird-issue-importer firebird-issue-importer commented Oct 9, 2009

Modified by: @hvlad

assignee: Vlad Khorsun [ hvlad ]

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@firebird-issue-importer firebird-issue-importer commented Oct 12, 2009

Modified by: @hvlad

status: Open [ 1 ] => Resolved [ 5 ]

resolution: Fixed [ 1 ]

Fix Version: 2.5 RC1 [ 10362 ]

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@firebird-issue-importer firebird-issue-importer commented Nov 12, 2009

Modified by: @pcisar

status: Resolved [ 5 ] => Closed [ 6 ]

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@firebird-issue-importer firebird-issue-importer commented Jan 18, 2016

Modified by: @pavel-zotov

QA Status: No test

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@firebird-issue-importer firebird-issue-importer commented May 24, 2016

Modified by: @pavel-zotov

status: Closed [ 6 ] => Closed [ 6 ]

QA Status: No test => Cannot be tested

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