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Jun 2, 2010 · 88. An index is used to speed up the performance of queries. It does this by reducing the number of database data pages that have to be visited/scanned. In SQL Server, a clustered index determines the physical order of data in a table. There can be only one clustered index per table (the clustered index IS the table).
Example 2 - indexing. Given our sample database of r = 5,000,000 records with an index record length of R = 54 bytes and using the default block size B = 1,024 bytes. The blocking factor of the index would be bfr = (B/R) = 1024/54 = 18 records per disk block. The total number of blocks required to hold the index is N = (r/bfr) = 5000000/18 ...
Jul 6, 2011 · For traditional SQL Server, the filters you define in your 'Where' clause should persuade the engine to use any relevant indices... Provided the engine's execution plan can efficiently identify how to read the information (whether a full table scan or an indexed scan) - it must compare the two before executing the statement proper, as part of its built-in performance optimiser.
Jan 19, 2017 · Created a stored procedure to list indexes for a table in database in SQL Server. create procedure _ListIndexes(@tableName nvarchar(200)) as begin /* exec _ListIndexes '<YOUR TABLE NAME>' */ SELECT DB_NAME(DB_ID()) as DBName,SCH.name + '.' + TBL.name AS TableName,IDX.name as IndexName, IDX.type_desc AS IndexType,COL.Name as ColumnName,IC.*
Mar 1, 2011 · 1. An index is best placed on a value that is as unique as possible. It is, for instance, useless to place an index on a column where 50% of that column is value 'A', and another 50% of the column has a value 'B'. That way, the table will scan at least 50% of the records before selecting the right values.
E.g. a x-y composite B-tree index would: efficiently speed up: x = 1 and y = 2: equalities in both columns. x = 1 and y > 2: equality on first column plus inequality on the second column. x = 1 ORDER BY y: order by immediately after previous equalities (x is before y on column index order) is efficiently sped up.
Aug 16, 2012 · Over-indexing in SQL Server can be worse than not having any indexes. Don't start out with too many indices to begin with! Don't start out with too many indices to begin with! Only establish good clustered PK and foreign key nonclustered indices - that's all - then observe, measure, optimize & repeat that cycle.
DBMS use indexes to support unique constraints. Always create indexes to enforce unique constraints; they (the constraints) are crucial to the correct operation of your database. Where you have a choice of how to create the index on multiple columns, put the column that will always be referenced in the queries ahead of other fields — usually.
Aug 25, 2010 · In MySQL InnoDB, there are two types of index. Primary key which is called clustered index. Index key words are stored with real record data in the B+Tree leaf node. Secondary key which is non clustered index. These index only store primary key's key words along with their own index key words in the B+Tree leaf node.
MySQL does not support analytic/"windowing"/OLAP-type functions, so this is as elegant as it gets! In Oracle or MS SQL-Server you could write: select id, name, rating, row_number over (partition by id order by rating desc) "index" from users order by rating desc –
