![]() ![]() Your application may require an index based on a lengthy string, or even worse, a concatenation of two strings, or of a string and one or two integers. – when you have >= SQL 2014 you can create NC indexes in the CREATE TABLE statment.Using Hash Keys instead of String Indexes – indexes on temp tables makes usually (!) only sense, if you use the #temp more than once, since an index creation needs a table scan (and if you use the #temp only once then it would be faster, when you make scan in the SELECT and skip the writes for the index creation) Sorry, but I stand by my statement that indexes on temp tables are way more frequently a waste than a benefit. But MANY times I have seen that be a RBAR (Row By Agonizing Row) process that was actually able to be rolled up into a fully SET based operation and become orders of magnitude more efficient because of that. But guess what – the optimizer can and often does insert it’s own sort of the temp table to facilitate that same merge join.Ģ) LOTS of iterative hits on the same object by some changing field value. That huge read-sort-write process (especially for clustered indexes) to create the index just wastes effort for no overall gain.ġ) A clustered index that allows the optimizer to use a merge join on a very large table. Then (almost) ALWAYS there is just ONE use of that temp table for a join and then it’s usefulness is done. But in doing so you have to read the table entirely and then build the index – all taking time and resources. Or perhaps they use a little bit of grey matter and decide to index some field that will be used for a join or a predicate. A common one is to put an identity on the table and a primary key on that (clustered by default of course). SQL Server retrieves the data row by searching the clustered index using the clustered index key stored in the leaf row of the nonclustered index. It is only added when required to make the clustered key unique for use in nonclustered indexes. This four-byte value is not visible to users. If the clustered index is not a unique index, SQL Server makes any duplicate keys unique by adding an internally generated value called a uniqueifier. If the table has a clustered index, or the index is on an indexed view, the row locator is the clustered index key for the row. The whole pointer is known as a Row ID (RID). The pointer is built from the file identifier (ID), page number, and number of the row on the page. If the table is a heap, which means it does not have a clustered index, the row locator is a pointer to the row. Or a clustered index key for a row, as described in the following: ![]() Row locators in nonclustered index rows are…. (my mistake was on the size of the uniquifier, it’s 4 bytes) Therefore they are always either implicitly or explicitly unique. ON tc.UserId = tp.OwnerUserId - is the same tu.id due to inner join aboveĮrik, SQL Server must be able to use a clustered index seek to locate a single row. The query plan has a leading short on one of the tables, so the cluster index can precalculate that sort,ģb,or note that user ID is common to both joins, and make that the leading column of the indexĬREATE unique CLUSTERED INDEX cx_tempposts ON #TempPosts ( OwnerUserId, Id ) ĬREATE unique CLUSTERED INDEX cx_tempusers ON #TempUsers (Id) ĬREATE unique CLUSTERED INDEX cx_tempcomments ON #TempComments (UserId, PostId) You don’t have to index the way of the present table, It’s your temp and in this case you perfectly know the query pattern it is going to be use for.ģa. The are a couple of things to keep in mind.ġ.all cluster indexes are unique, so adding the extra columns to make the index unique is a free action, if they are 8 bytes or less (the size of the uniqueafier, if I remember correctly), also telling sql server that some joins are 1:n, not potentially n:n cash change the plan.Ģ. ![]()
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