Databases
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    <blockquote> <p>1) Is it true that for every index a copy of the table is created?</p> </blockquote> <p>Partially. The columns that are indexed by a particular (non-clustered) index will be "copied" into index data structure (typically a B-Tree), but the rest of the columns will not. BTW, some RDMBSes support a form of index compression (such as Oracle, not sure about MS SQL Server), which may significantly "shrink" a footprint of a low-selectivity index.</p> <p>As for a clustered index, the table and the index are actually the same data structure. So if all you have is a single index (i.e. just a primary key), making it clustered will typically lower the space required compared to a classical B-Tree / table heap combination that you get from a non-clustered index.</p> <p><strong>However</strong>, clustered index increases "copying" that is going on for <strong>all other</strong> (i.e. non-clustered) indexes. From MSDN:</p> <p><em>"The key values from the clustered index are used by all non-clustered indexes as lookup keys and therefore are stored in each nonclustered index leaf entry."</em></p> <p>If you cannot escape having many indexes, at least get rid of the cluster and use only non-clustered indexes.</p> <blockquote> <p>2) 32 indexes add only 1.5 seconds for the same INSERT command. Does this average remain for 10,000 rows? or for 80 indexes?</p> </blockquote> <p>With only 3,400 rows (or 10,000 for that matter), I suspect you are still well within the confines of the cache, which may mask the performance problem you'll probably have maintaining all these indexes for a larger data set. As always, you won't know for sure until you actually benchmark with representative amounts of data...</p> <h2>--- EDIT ---</h2> <blockquote> <p>3)when we have a Foreign key on a column SQL Server automatically create non-cluster index on it.this Index useful for quering or Joins?</p> </blockquote> <p>It may or may not increase the performance, depending on how particular query is constructed. It may also be significant for cascading referential integrity (ON DELETE/UPDATE CASCADE).</p> <p>If you can live without this index, removing it will increase the performance of modifications, as with removing any other index.</p>
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    1. POWhat is the impact of CRUD operations and JOINS with multiple indexes?
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    1. USBranko Dimitrijevic
    1. USBranko Dimitrijevic
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    1. COthanks.I create a test table in my database in join it with main table on column that has not any index and in execution plan I got missing index message that suggest index on column in main table.What is your idea?
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      1. USArian
    2. CO@Nima When JOIN is implemented as a NESTED LOOP, the "outer" table is driving the "inner" table and the inner table better have an index if you care about performance. When **both** parent PRIMARY KEY and child FOREIGH KEY have indexes, the optimizer will typically choose whatever table is more "selective" (i.e. returns fewer rows) as "outer" and the other table then automatically becomes "inner". If you have **only** parent PRIMARY KEY index, then you are making it very expensive for child table to become "inner" and thus limiting optimizer's choices (which may or may not be a problem).
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      1. USBranko Dimitrijevic
 

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