How does index work in oracle

You can create an index for a table to improve the performance of queries issued against the corresponding table. You can also create an index for a cluster. You can create a composite index on multiple columns up to a maximum of 32 columns. A composite index key cannot exceed roughly one-half minus some overhead of the available space in the data block.

In this example, an index is created for a single column, to speed up queries that test that column:. In this example, the index applies to two columns, to speed up queries that test either the first column or both columns:. An index on the ENAME column itself would not speed up this operation, and it might be slow to call the function for each result row.

A function-based index precomputes the result of the function for each column value, speeding up queries that use the function for searching or sorting:. Domain indexes are appropriate for special-purpose applications implemented using data cartridges.

The domain index helps to manipulate complex data, such as spatial, time-series, audio, or video data. Oracle supplies a number of specialized data cartridges to help manage these kinds of complex data. So, if you need to create a search engine, or a geographic information system, you can do much of the work simply by creating the right kind of index.

A function-based index is an index built on an expression. It extends your indexing capabilities beyond indexing on a column. A function-based index increases the variety of ways in which you can access data.

Function-based indexes also support linguistic sorts based on collation keys, efficient linguistic collation of SQL statements, and case-insensitive sorts. Like other indexes, function-based indexes improve query performance.

For example, if you need to access a computationally complex expression often, then you can store it in an index. Then when you need to access the expression, it is already computed. You can find a detailed description of the advantages of function-based indexes in "Advantages of Function-Based Indexes". Function-based indexes have all of the same properties as indexes on columns.

However, unlike indexes on columns which can be used by both cost-based and rule-based optimization, function-based indexes can be used by only by cost-based optimization. Other restrictions on function-based indexes are described in "Restrictions for Function-Based Indexes".

For more information on function-based indexes, see Oracle9i Database Concepts. For information on creating function-based indexes, see Oracle9i Database Administrator's Guide. The optimizer can estimate how many rows are selected by expressions more accurately if the expressions are materialized in a function-based index. Restrictions on Global Partitioned Indexes Global partitioned indexes are subject to the following restrictions:.

The only property you can specify for hash partitions is tablespace storage. The number of repetitions of this clause determines the number of partitions. Oracle Database automatically maintains local index partitioning as the underlying table is repartitioned. If you omit partition , then Oracle Database generates a name that is consistent with the corresponding table partition.

You cannot specify key compression for an index partition unless you have specified key compression for the index. You can optionally specify tablespace storage for one or more individual partitions. If you do not specify tablespace storage either here or in the STORE IN clause, then the database stores each index partition in the same tablespace as the corresponding table partition.

The number of tablespaces need not equal the number of index partitions. If the number of index partitions is greater than the number of tablespaces, then the database cycles through the names of the tablespaces. It lets you specify one or more default tablespaces across which Oracle Database will distribute all index hash subpartitions.

It lets you specify one or more tablespaces across which Oracle Database will distribute all the index hash subpartitions. If you omit subpartition , then the database generates a name that is consistent with the corresponding table subpartition. The number of tablespaces need not equal the number of index subpartitions. If the number of index subpartitions is greater than the number of tablespaces, then the database cycles through the names of the tablespaces.

If you also do not specify tablespace storage for index , then the database stores the subpartition in the same tablespace as the corresponding table subpartition. Creating a domain index requires a number of preceding operations. You must first create an implementation type for an indextype. You must also create a functional implementation and then create an operator that uses the function.

Next you create an indextype, which associates the implementation type with the operator. Finally, you create the domain index using this clause. Please refer to Appendix E, "Examples" , which contains an example of creating a simple domain index, including all of these operations. You can define multiple domain indexes on a single column only if the underlying indextypes are different and the indextypes support a disjoint set of user-defined operators.

Restrictions on Domain Indexes Domain indexes are subject to the following restrictions:. This name should be a valid schema object that has already been created. If you have installed Oracle Text, you can use various built-in indextypes to create Oracle Text domain indexes. For a nonpartitioned domain index, Oracle Database passes the explicit or default degree of parallelism to the ODCIIndexCreate cartridge routine, which in turn establishes parallelism for the index.

The maximum length of the parameter string is characters. When you specify this clause at the top level of the syntax, the parameters become the default parameters for the index partitions. A bitmap join index is defined on a single table. For an index key made up of dimension table columns, it stores the fact table rowids corresponding to that key. In a data warehousing environment, the table on which the index is defined is commonly referred to as a fact table , and the tables with which this table is joined are commonly referred to as dimension tables.

However, a star schema is not a requirement for creating a join index. ON In the ON clause, first specify the fact table, and then inside the parentheses specify the columns of the dimension tables on which the index is defined. If a dimension table has a composite primary key, each column in the primary key must be part of the join.

Creating a Cluster Index: Example To create an index for the personnel cluster, which was created in "Creating a Cluster: Example" , issue the following statement:. No index columns are specified, because cluster indexes are automatically built on all the columns of the cluster key. For cluster indexes, all rows are indexed. Such an index would greatly improve the performance of queries that select from the table based on, for example, the square footage of a warehouse, as shown in this statement:.

See the "Prerequisites" for the privileges and parameter settings required when creating function-based indexes. To ensure that Oracle Database will use the index rather than performing a full table scan, be sure that the value returned by the function is not null in subsequent queries.

For example, this statement is guaranteed to use the index:. Creating a Function-based Index on a Type Method: Example This example entails an object type rectangle containing two number attributes: length and width. The area method computes the area of the rectangle. Using a Function-based Index to Define Conditional Uniqueness: Example The following statement creates a unique function-based index on the oe.

Oracle Database will distribute the hash partitions between the tbs1 and tbs2 tablespaces:. The creator of the index must have quota on the tablespaces specified. The following creates a bitmap join index on the table oe. In this example, the user must have quota on tablespaces specified. The sample table pm. You can build an index on attributes of the declared type of a substitutable column. The following example uses the table books , which is created in "Substitutable Table and Column Examples".

The statement creates an index on the salary attribute of all employee authors in the books table:. The target type in the argument of the TREAT function must be the type that added the attribute being referenced. If this condition is not satisfied, then Oracle Database interprets the TREAT function as any functional expression and creates the index as a function-based index.

Another option is that, you could just go to the Index section of the book and then find what you are looking for by using the Name of the entity you are looking in this instance, German Shepherds and also looking at the page number to quickly find what you are looking for. In Database, the page number is referred to as a pointer which directs the database to the address on the disk where entity is located.

In short, an index is a data structure that stores the values for a specific column in a table so as to speed up query search. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow.

Learn more. How does database indexing work? Asked 13 years, 3 months ago. Active 3 months ago. Viewed k times. Improve this question. TRiG 9, 6 6 gold badges 52 52 silver badges bronze badges. Xenph Yan Xenph Yan Add a comment. Active Oldest Votes. Why is it needed? What is indexing? How does it work?

The schema for an index on the firstName field is outlined below; Field name Data type Size on disk firstName Char 50 50 bytes record pointer Special 4 bytes Note : Pointers in MySQL are 2, 3, 4 or 5 bytes in length depending on the size of the table. When should it be used?

Improve this answer. Ricola 2, 8 8 silver badges 17 17 bronze badges. AbhishekShivkumar:Great question! I think the index table will have as many rows as there are in the data table. In this case,it's worthless to index on such a column.

Hope I'm correct : — Saurabh Patil. I think there are a few typos in this answer, for example, in the sentence: "a far cry from the , block accesses required by the non-indexed table. There seems to be a couple of other inaccuracies too : — jcm. Creating an index on a field in a table creates another data structure which holds the field value, and pointer to the record it relates to. Show 28 more comments. Classic example "Index in Books" Consider a "Book" of pages, divided by 10 Chapters, each section with pages.

Simple, huh? This analogy is known as "Full Table Scan" in database world. Things are simple in schools, isn't it? Sankarganesh Eswaran Sankarganesh Eswaran 8, 2 2 gold badges 19 19 silver badges 21 21 bronze badges. This makes me think Library or Grocery Store Could you image not having an index at a grocery store? Where's The Beef?!? Oh its next to the Restrooms, a mop, and makeup — JayRizzo.

Indices usually go at the back of books, while a table of contents goes in the front. But, that makes the analogy even better, since column order shouldn't matter. I still do not exactly understand, so if there are n unique words how would index help me? If so it takes a lot of time to find that pointer maybe even same time then just scroll everything and find it in a default way — D0mm. Show 1 more comment.

Let's note that "just a data structure" doesn't mean "additional to the data". Some times it is e. The first time I read this it was very helpful to me. Thank you. In certain scenarios a heap is more helpful than a table with indexes, e. Also, a differentiation between clustered and non-clustered indexes is rather important.

Community Bot 1 1 1 silver badge. Der U Der U 3, 1 1 gold badge 10 10 silver badges 12 12 bronze badges. I think, these indexing issues can be resolved by maintaining two different databases, just as Master and Slave. Where Master can be used to insert or update records. Without indexing.