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I was asked this question at an interview and had no answer. Can anyone here explain?

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This article is a good starting point. –  JosephStyons Oct 15 '08 at 18:52
    
Today I read a very nice article about SQL Server lock escalation: sqlservercentral.com/blogs/aschenbrenner/2014/02/25/… try it out. –  ddaniel Mar 3 at 12:28
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migrated from stackoverflow.com Feb 13 '12 at 21:39

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3 Answers

up vote 25 down vote accepted

DB locks can exist on rows, pages or whole tables or indexes. When a transaction is in progress, the locks held by the transaction take up resources. Lock escalation is where the system consolidates multiple locks into a higher level one (for example consolidating multiple row locks to a page or multiple pages to a whole table) typically to recover resources taken up by large numbers of fine-grained locks.

It will do this automatically, although you can set flags on the tables (see ALTER TABLE in the books on line) to control the policy for lock escalation on that particular table. In particular, premature or overly eager lock escalation used to be a problem on older versions of Sybase and SQL Server when you had two processes writing separate rows into the same page concurrently. If you go back far enough (IIRC SQL Server 6.5) SQL Server didn't actually have row locking but could only lock tables or pages. Where this happened, you could get contention between inserts of records in the same page; often you would put a clustered index on the table so new inserts went to different pages.

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It should also be noted that we have no control over when the server escalates. –  Josef Oct 15 '08 at 20:15
    
We do now. It's call T1211. –  Joshua Apr 4 '13 at 16:17
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It's a method for reducing system overhead, by converting many fine grained locks to fewer coarse grained ones. More detailed information can be found here and here.

For example, if you have many (usually hundreds or more) locks on specific rows in a table, once you exceed your maximum allowed number of locks, these might be exchanged for a lock on the table, or on a range of rows in the table.

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Lock escalation means the conversion of a lock to a more restrictive mode. This is most often seen in databases. A query might have a resource locked for "shared" and escalate it to "exclusive" to perform an update.

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