The most common scenario for using reindex is when the data has changed significantly, and there are now existing pages that are inefficiently using space. This article covered the main considerations for creating a database index that improves performance instead of slowing it down:. Now that you've optimized your query performance, it's time to speed up your SQL workflow. Arctype's collaborative SQL client allows you to easily share databases, queries, and visualizations with anyone.
If you liked this post, join the Arctype Newsletter and receive the best articles from our blog once a month. Table of contents. Type Performance When to use? Longer insertion time. Indexing data types that map multiple values to one row i. For guidelines that help you choose which columns to index, see the Oracle7 Server Application Developer's Guide.
By default, Oracle locks data at the row level rather than the table level. Row-level locking allows multiple users to access different rows in the same table concurrently. The resulting performance gain is significant. You can specify table-level locking, but it lessens the effectiveness of the transaction processing option. Applications that do online transaction processing benefit most from row-level locking. If your application relies on table-level locking, modify it to take advantage of row-level locking.
In general, avoid explicit table-level locking. Eliminating unnecessary parsing requires correct handling of cursors and selective use of the following cursor management options:. These options affect implicit and explicit cursors, the cursor cache, and private SQL areas.
See "Using the Oracle Communications Area". Recall that there are two types of cursors: implicit and explicit see page 2 - 7.
Oracle implicitly declares a cursor for all data definition and data manipulation statements. However, for queries that return more than one row, you must explicitly declare a cursor or use host arrays. How you handle the opening and closing of explicit cursors affects performance. If you need to reevaluate the active set, simply reopen the cursor. The OPEN statement will use any new host-variable values.
You can save processing time if you do not close the cursor first. Note : To make performance tuning easier, the precompiler lets you reopen an already open cursor. For example, your program should close all cursors before exiting.
With the first way, beware of unnecessary parsing. A SQL statement need be parsed only once unless you change its makeup. Declaring an explicit cursor gives you maximum control over parsing. When a data manipulation statement is executed, its associated cursor is linked to an entry in the cursor cache.
When you already have a table where the records are sorted ascending by e. One of the rare scenarios where a heap table can be a good practice is when the row identifier is smaller than the clustered index. Any SQL Server table configuration where performance suffers due to excessive, improper, or missing indexes is considered to be poor indexing. If indexes are not properly created, SQL Server has to go through more records in order to retrieve the data requested by a query.
Therefore, it uses more hardware resources processor, memory, disk, and network and obtaining the data lasts longer. A table without a clustered index can also be considered as a poor indexing practice. Execution of a SELECT statement, inserting, updating, and deleting records is in most cases slower on a heap table than on a clustered one.
When you create a new table with a primary key in a SQL Server database, a unique clustered index is automatically created on the primary key column. Although this default action is acceptable in most cases, this might not be the optimal clustered index.
The column used for a clustered index should be a unique, identity, or primary key, or any other column where the value is increased for each new entry. As clustered indexes sort the records based on the value, using a column already ordered ascending, such as an identity column, is a good solution. If a column where new values are not higher than previous is used for a clustered index, adding each new row would require re-ordering, i.
Improve this question. Nicolas Kaiser 1 1 gold badge 1 1 silver badge 9 9 bronze badges. I will do tomorrow : — paweloque. How about updates? Add a comment. Active Oldest Votes. In general, consider creating an index on a column in any of the following situations: The indexed columns are queried frequently and return a small percentage of the total number of rows in the table. Improve this answer. Leigh Riffel Leigh Riffel Community Bot 1.
Gaius Gaius 11k 3 3 gold badges 28 28 silver badges 63 63 bronze badges. That page is about 'selecting an index strategy': Limit the Number of Indexes for Each Table The more indexes, the more overhead is incurred as the table is altered.
Sangram Sangram 9 1 1 bronze badge. From the question: "Obviously keeping several different indices have a negative impact on insert and delete performance. How about query performance" — Jack Douglas. Sign up or log in Sign up using Google.
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