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Ghost Records


Ghost Records

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Steve Jones
Steve Jones
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Comments posted to this topic are about the item Ghost Records

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Peter Trast
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First to answer and got it right! Never heard of it but the right answer was perfectly logical.

Great question!

Peter Trast
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M&M
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I remember reading about this in an article.

M&M
khullargirish02
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When rows are marked as deleted after a DML operation and the transaction is committed, the rows become Ghost records.

I got it right Smile
Kwex
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Nice to know! DB2 behaves quite differently, because it just re-uses the page after records have been marked as deleted. It doesn't need an extra background process (ghost exorciser) to make a physical delete.

Not sure why SQL Server adopted this approach :-)

Kwex.
dave.clark
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I believe it is done for performance reasons. Deleting data can occur faster and rollbacks would be faster. Then a process can later update clear up the ghost records.
chrisfradenburg
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As to why something goes through and cleans up the ghost records instead of just allowing the space get reused, that way the data doesn't remain on disk so it doesn't accidentally read by something. Or purposefully read by someone for that matter.
GilaMonster
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cfradenburg (6/2/2011)
As to why something goes through and cleans up the ghost records instead of just allowing the space get reused, that way the data doesn't remain on disk so it doesn't accidentally read by something. Or purposefully read by someone for that matter.


Ghost cleanup doesn't overwrite the ghosted rows. A direct read of the data file can see deleted data well after the ghosted rows are removed.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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chrisfradenburg
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Interesting. What does the ghost cleanup process do then?
SQLRNNR
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GilaMonster (6/2/2011)
cfradenburg (6/2/2011)
As to why something goes through and cleans up the ghost records instead of just allowing the space get reused, that way the data doesn't remain on disk so it doesn't accidentally read by something. Or purposefully read by someone for that matter.


Ghost cleanup doesn't overwrite the ghosted rows. A direct read of the data file can see deleted data well after the ghosted rows are removed.


Sounds like a fun experiment...



Jason AKA CirqueDeSQLeil
I have given a name to my pain...
MCM SQL Server, MVP


SQL RNNR

Posting Performance Based Questions - Gail Shaw

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