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QuestionRID|Key lookup vs Index Seek? [modified] Pin
devvvy5-Jul-11 17:23
devvvy5-Jul-11 17:23 
Hi
I understand what index seek means - but not sure about distinction between Index Seek, RID Lookup (Heap based what the hell...) and Key Lookup (clustered index lookup)

From this reference: [^]

RID Lookup is a form of a bookmark index lookup on a table without a clustered index (a heap).
> So both clustered and non-clustered is on the disk and SQL server actually build another index in memory/heap?! And all RID's (Row ID) are stored in volatile memory!?

key lookup is a bookmark lookup on a table with a clustered index.
> So how is it different from Clustered Index Seek?

I'm a bit unclear on seek vs lookup, but seems like "non-clustered seek" (non-clustered ONLY)[^] fetches only the RID, database engine still need to perform a "lookup" by RID to fetch the actual values of selected columns.

For "clustered index seek", leaf is actual data so there should never be lookup for rows fetched via "clustered index seek" (thus no idea what a "Key Lookup" is about)

Similar question to http://www.web-shelf.com/microsoft.public.sqlserver.relationalserver.performance/index-seek-with-lookup-vs-index-seek-with-include-thread-ID1202.aspx[^]

Adding to confusion, from MSDN[^]- so RID is in heap (where? All rows RID in heap/memory!?) and Key lookup is done via clustered index on the disk? HEAP (aka "Base Table) is not Clustered Index which is stored on disk.
RID Lookup is a bookmark lookup on a heap using a supplied row identifier (RID). The Argument column contains the bookmark label used to look up the row in the table and the name of the table in which the row is looked up. RID Lookup is always accompanied by a NESTED LOOP JOIN.

Key Lookup operator is a bookmark lookup on a table with a clustered index. The Argument column contains the name of the clustered index and the clustering key used to look up the row in the clustered index. Key Lookup is always accompanied by a Nested Loops operator. If the WITH PREFETCH clause appears in the Argument column, the query processor has determined that it is optimal to use asynchronous prefetching (read-ahead) when looking up bookmarks in the clustered index. For more information about read-ahead, see Reading Pages.


Thanks
dev
modified on Tuesday, July 5, 2011 11:46 PM

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