11/15/2022 0 Comments Query processing in dbms wikipedia![]() ![]() Primary index, equality on nonkey: The difference between equality on key and nonkey is that in this, we can fetch multiple records.The equality comparison is performed on the key attribute carrying a primary key. Primary index, equality on a key: We use the index to retrieve a single record that satisfies the equality condition for making the selection.There are following algorithms that use the index in query processing: These paths allow locating and accessing the data in the file. Such index structures are known as access paths. The index-based search algorithms are known as Index scans. But other algorithms are not applicable in all types of cases. This algorithm does not care about the nature of selection, availability of indices, or the file sequence. However, linear search is the slowest algorithm used for searching, but it is applicable in all types of cases. If the blocks in the file are not stored in contiguous order, then it needs some extra seeks. For accessing the first block of a file, it needs an initial seek. Linear Search: In a linear search, the system scans each record to test whether satisfying the given selection condition.When the selection operation is performed on a relation whose tuples are stored in one file, it uses the following algorithms: In RDBMS or relational database systems, the file scan reads a relation only if the whole relation is stored in one file only. Let's see how selection using a file scan is performed. It is the lowest-level operator used in query processing. File scans are the search algorithms that are used for locating and accessing the data. Generally, the selection operation is performed by the file scan. In this section, we will understand how the selection operation is performed in the query execution plan. In the previous section, we understood that estimating the cost of a query plan should be done by measuring the total resource consumption. Next → ← prev Selection Operation in Query Processing ![]()
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