Oracle what type of index

It allows transactions in between. The mechanism has been quite improved with 11g and 12c. Indexes increase the performance of a select query, they can also decrease the performance of data manipulation. Though the extra space occupied by indexes is also a consideration, it may not matter much since the cost of data storage has declined substantially.

An unusable index is ignored by the optimizer in deciding the explain plan. It is also not maintained by DML i. There could be several reasons for the index being in an unusable state. You did the rebuild of the table but did not rebuild the index, then the index will be in an unusable state. One other reason to make an index unusable is to improve bulk load performance. Another reason might be optimizer picking up the wrong index every time and time is critical so you may decide to make it unusable.

An unusable index or index partition must be rebuilt, or dropped and re-created before it can be used. Truncating a table makes an unusable index valid. Beginning with Oracle Database 11g Release 2, when you make an existing index unusable, its index segment is dropped. For nonpartitioned indexes, the optimizer does not consider any unusable indexes when creating an access plan for SELECT statements.

If any unusable indexes or index partitions are present, any DML statements that would cause those indexes or index partitions to be updated are terminated with an error. For SELECT statements, if an unusable index or unusable index partition is present but the optimizer does not choose to use it for the access plan, the statement proceeds.

However, if the optimizer does choose to use the unusable index or unusable index partition, the statement terminates with an error. This course gives us tricks and lessons on how to effectively use it and become a productive sql developer Oracle SQL Performance Tuning Masterclass : Performance tuning is one of the critical and most sought skills.

This is a good course to learn about it and start doing sql performance tuning. Skip to primary navigation Skip to main content Skip to primary sidebar Home » Oracle » Oracle Database » Oracle Indexes and types of indexes in oracle with example. Folders containing employee information are inserted randomly in the boxes. The folder for employee Whalen ID is 10 folders up from the bottom of box 1, whereas the folder for King ID is at the bottom of box 3. To locate a folder, the manager looks at every folder in box 1 from bottom to top, and then moves from box to box until the folder is found.

To speed access, the manager could create an index that sequentially lists every employee ID with its folder location:. Similarly, the manager could create separate indexes for employee last names, department IDs, and so on. The indexed columns are queried frequently and return a small percentage of the total number of rows in the table. A referential integrity constraint exists on the indexed column or columns. The index is a means to avoid a full table lock that would otherwise be required if you update the parent table primary key , merge into the parent table, or delete from the parent table.

A unique key constraint will be placed on the table and you want to manually specify the index and all index options. Indexes are schema objects that are logically and physically independent of the data in the objects with which they are associated.

Thus, an index can be dropped or created without physically affecting the table for the index. The absence or presence of an index does not require a change in the wording of any SQL statement. An index is a fast access path to a single row of data.

It affects only the speed of execution. Given a data value that has been indexed, the index points directly to the location of the rows containing that value. The database automatically maintains and uses indexes after they are created. The database also automatically reflects changes to data, such as adding, updating, and deleting rows, in all relevant indexes with no additional actions required by users.

Retrieval performance of indexed data remains almost constant, even as rows are inserted. However, the presence of many indexes on a table degrades DML performance because the database must also update the indexes.

Indexes are usable default or unusable. An unusable index is not maintained by DML operations and is ignored by the optimizer. An unusable index can improve the performance of bulk loads.

Instead of dropping an index and later re-creating it, you can make the index unusable and then rebuild it. Unusable indexes and index partitions do not consume space. When you make a usable index unusable, the database drops its index segment.

Indexes are visible default or invisible. An invisible index is maintained by DML operations and is not used by default by the optimizer. Making an index invisible is an alternative to making it unusable or dropping it. Invisible indexes are especially useful for testing the removal of an index before dropping it or using indexes temporarily without affecting the overall application.

Oracle Database Performance Tuning Guide to learn how to tune indexes. A key is a set of columns or expressions on which you can build an index. Although the terms are often used interchangeably, indexes and keys are different. Indexes are structures stored in the database that users manage using SQL statements. Keys are strictly a logical concept. A composite index , also called a concatenated index , is an index on multiple columns in a table. Columns in a composite index should appear in the order that makes the most sense for the queries that will retrieve data and need not be adjacent in the table.

Therefore, the order of the columns used in the definition is important. In general, the most commonly accessed columns go first. You create an index with the following column order:. Multiple indexes can exist for the same table if the permutation of columns differs for each index. You can create multiple indexes using the same columns if you specify distinctly different permutations of the columns.

For example, the following SQL statements specify valid permutations:. Indexes can be unique or nonunique. Unique indexes guarantee that no two rows of a table have duplicate values in the key column or columns. For example, no two employees can have the same employee ID. Thus, in a unique index, one rowid exists for each data value. The data in the leaf blocks is sorted only by key.

Nonunique indexes permit duplicates values in the indexed column or columns. For a nonunique index, the rowid is included in the key in sorted order, so nonunique indexes are sorted by the index key and rowid ascending. Oracle Database does not index table rows in which all key columns are null , except for bitmap indexes or when the cluster key column value is null. Oracle Database provides several indexing schemes, which provide complementary performance functionality.

The indexes can be categorized as follows:. These indexes are the standard index type. They are excellent for primary key and highly-selective indexes. Used as concatenated indexes, B-tree indexes can retrieve data sorted by the indexed columns. B-tree indexes have the following subtypes:.

An index-organized table differs from a heap-organized because the data is itself the index. See "Overview of Index-Organized Tables". In this type of index, the bytes of the index key are reversed, for example, is stored as The reversal of bytes spreads out inserts into the index over many blocks. See "Reverse Key Indexes". This type of index stores data on a particular column or columns in descending order. See "Ascending and Descending Indexes". This type of index is used to index a table cluster key.

Instead of pointing to a row, the key points to the block that contains rows related to the cluster key. See "Overview of Indexed Clusters". In a bitmap index, an index entry uses a bitmap to point to multiple rows.

In contrast, a B-tree index entry points to a single row. A bitmap join index is a bitmap index for the join of two or more tables. See "Bitmap Indexes". This type of index includes columns that are either transformed by a function, such as the UPPER function, or included in an expression. B-tree or bitmap indexes can be function-based. See "Function-Based Indexes". This type of index is created by a user for data in an application-specific domain. The physical index need not use a traditional index structure and can be stored either in the Oracle database as tables or externally as a file.

See "Application Domain Indexes". B-trees, short for balanced trees , are the most common type of database index. A B-tree index is an ordered list of values divided into ranges.

By associating a key with a row or range of rows, B-trees provide excellent retrieval performance for a wide range of queries, including exact match and range searches. Figure illustrates the structure of a B-tree index. A B-tree index has two types of blocks: branch blocks for searching and leaf blocks that store values.

The upper-level branch blocks of a B-tree index contain index data that points to lower-level index blocks. In Figure , the root branch block has an entry , which points to the leftmost block in the next branch level. This branch block contains entries such as and Each of these entries points to a leaf block that contains key values that fall in the range. A B-tree index is balanced because all leaf blocks automatically stay at the same depth. Thus, retrieval of any record from anywhere in the index takes approximately the same amount of time.

The height of the index is the number of blocks required to go from the root block to a leaf block. The branch level is the height minus 1. In Figure , the index has a height of 3 and a branch level of 2. Branch blocks store the minimum key prefix needed to make a branching decision between two keys. This technique enables the database to fit as much data as possible on each branch block. The branch blocks contain a pointer to the child block containing the key.

The number of keys and pointers is limited by the block size. The leaf blocks contain every indexed data value and a corresponding rowid used to locate the actual row. Each entry is sorted by key, rowid. Within a leaf block, a key and rowid is linked to its left and right sibling entries. The leaf blocks themselves are also doubly linked.

In Figure the leftmost leaf block is linked to the second leaf block In an index scan , the database retrieves a row by traversing the index, using the indexed column values specified by the statement. This is the basic principle behind Oracle Database indexes. If a SQL statement accesses only indexed columns, then the database reads values directly from the index rather than from the table. If the statement accesses columns in addition to the indexed columns, then the database uses rowids to find the rows in the table.

Typically, the database retrieves table data by alternately reading an index block and then a table block. In a full index scan , the database reads the entire index in order. A full index scan is available if a predicate WHERE clause in the SQL statement references a column in the index, and in some circumstances when no predicate is specified.

A full scan can eliminate sorting because the data is ordered by index key. Oracle Database performs a full scan of the index, reading it in sorted order ordered by department ID and last name and filtering on the salary attribute. In this way, the database scans a set of data smaller than the employees table, which contains more columns than are included in the query, and avoids sorting the data.

A fast full index scan is a full index scan in which the database accesses the data in the index itself without accessing the table, and the database reads the index blocks in no particular order. Fast full index scans are an alternative to a full table scan when both of the following conditions are met:. A row containing all nulls must not appear in the query result set. For this result to be guaranteed, at least one column in the index must have either:.

If the last name and salary are a composite key in an index, then a fast full index scan can read the index entries to obtain the requested information:. An index range scan is an ordered scan of an index that has the following characteristics:. One or more leading columns of an index are specified in conditions. The database commonly uses an index range scan to access selective data. The selectivity is the percentage of rows in the table that the query selects, with 0 meaning no rows and 1 meaning all rows.

A predicate becomes more selective as the value approaches 0 and less selective or more unselective as the value approaches 1. For example, a user queries employees whose last names begin with A. For example, two employees are named Austin, so two rowids are associated with the key Austin. An index range scan can be bounded on both sides, as in a query for departments with IDs between 10 and 40, or bounded on only one side, as in a query for IDs over To scan the index, the database moves backward or forward through the leaf blocks.

For example, a scan for IDs between 10 and 40 locates the first index leaf block that contains the lowest key value that is 10 or greater.

The scan then proceeds horizontally through the linked list of leaf nodes until it locates a value greater than In contrast to an index range scan, an index unique scan must have either 0 or 1 rowid associated with an index key.

The database performs a unique scan when a predicate references all of the columns in a UNIQUE index key using an equality operator. An index unique scan stops processing as soon as it finds the first record because no second record is possible. In this case, the database can use an index unique scan to locate the rowid for the employee whose ID is 5.

An index skip scan uses logical subindexes of a composite index. The database "skips" through a single index as if it were searching separate indexes. Skip scanning is beneficial if there are few distinct values in the leading column of a composite index and many distinct values in the nonleading key of the index. The database may choose an index skip scan when the leading column of the composite index is not specified in a query predicate.

For example, assume that you run the following query for a customer in the sh. Example shows a portion of the index entries. In a skip scan, the number of logical subindexes is determined by the number of distinct values in the leading column. In Example , the leading column has two possible values. The database logically splits the index into one subindex with the key F and a second subindex with the key M. When searching for the record for the customer whose email is Abbey company.

Conceptually, the database processes the query as follows:. The index clustering factor measures row order in relation to an indexed value such as employee last name. The more order that exists in row storage for this value, the lower the clustering factor. The index entries point to random table blocks, so the database may have to read and reread the same blocks over and over again to retrieve the data pointed to by the index.

Good for high-cardinality data. Good for low-cardinality data. Good for OLTP databases lots of updating. Good for data warehousing applications. Use a large amount of space. Use relatively little space. Easy to update. Difficult to update. Internally, a bitmap and a B-Tree indexes are very different, but functionally they are identical in that they serve to assist Oracle in retrieving rows faster than a full-table scan.

The basic differences between b-tree and bitmap indexes include:. The btree index does not say "bitmap". A b-tree index has index nodes based on data block size , it a tree form:. Index Creation. An Oracle bitmap index would look like. How many of our married customers live in the central or west regions?

Bitmap indexes can process this query with great efficiency by merely counting the number of ones in the resulting bitmap. To identify the specific customers who satisfy the criteria, the resulting bitmap would be used to access the table.

Bitmap Indexes and Nulls. Bitmap indexes include rows that have NULL values, unlike most other types of indexes. Any bitmap index can be used for this query because all table rows are indexed, including those that have NULL data.

Function-Based indexes give the ability to index computed columns and use theses indexes in a query, it allows you to have case insensitive searches or sorts, search complex equations and to extend the SQL language by implementing your own functions and operators and then searching on them. The main reason to use them are:. What is Clustered and Non-Clustered Index?

Clustered index is unique for any given table and we can have only one clustered index on a table. The leaf level of a clustered index is the actual data and the data is resorted in case of clustered index. In case of non-clustered index, the leaf level is actually a pointer to the data in rows so we can have as many non-clustered indexes as we can on the db.

Clustered indexes Physically stored in order ascending or descending Only one per table When a primary key is created a clustered index is automatically created as well.