The application software, called the cartridge , controls the structure and content of a domain index. The database interacts with the application to build, maintain, and search the domain index.
The index structure itself can be stored in the database as an index-organized table or externally as a file. Oracle Database stores index data in an index segment. Space available for index data in a data block is the data block size minus block overhead, entry overhead, rowid, and one length byte for each value indexed.
For ease of administration you can store an index in a separate tablespace from its table. For example, you may choose not to back up tablespaces containing only indexes, which can be rebuilt, and so decrease the time and storage required for backups. An index-organized table is a table stored in a variation of a B-tree index structure. In a heap-organized table , rows are inserted where they fit. In an index-organized table, rows are stored in an index defined on the primary key for the table.
Each index entry in the B-tree also stores the non-key column values. Thus, the index is the data, and the data is the index. Applications manipulate index-organized tables just like heap-organized tables, using SQL statements.
For an analogy of an index-organized table, suppose a human resources manager has a book case of cardboard boxes. Each box is labeled with a number—1, 2, 3, 4, and so on—but the boxes do not sit on the shelves in sequential order.
Instead, each box contains a pointer to the shelf location of the next box in the sequence. Folders containing employee records are stored in each box. The folders are sorted by employee ID.
The folder for employee is on top of , is on top of , and so on until box 1 is full. The next folder in the sequence is at the bottom of box 2. In this analogy, ordering folders by employee ID makes it possible to search efficiently for folders without having to maintain a separate index. Suppose a user requests the records for employees , , and Instead of searching an index in one step and retrieving the folders in a separate step, the manager can search the folders in sequential order and retrieve each folder as found.
Index-organized tables provide faster access to table rows by primary key or a valid prefix of the key. For example, the salary of employee is stored in the index row itself. Another benefit is the avoidance of the space overhead of a separate primary key index. Index-organized tables are useful when related pieces of data must be stored together or data must be physically stored in a specific order. Oracle Database Administrator's Guide to learn how to manage index-organized tables.
Oracle Database Performance Tuning Guide to learn how to use index-organized tables to improve performance. The database system performs all operations on index-organized tables by manipulating the B-tree index structure.
Table summarizes the differences between index-organized tables and heap-organized tables. The rowid uniquely identifies a row. Primary key constraint may optionally be defined.
Sequential full table scan returns all rows in some order. A full index scan or fast full index scan returns all rows in some order. Can be stored in a table cluster with other tables. Can contain virtual columns only relational heap tables are supported. Figure illustrates the structure of an index-organized departments table. The leaf blocks contain the rows of the table, ordered sequentially by primary key.
For example, the first value in the first leaf block shows a department ID of 20 , department name of Marketing , manager ID of , and location ID of An index-organized table stores all data in the same structure and does not need to store the rowid.
As shown in Figure , leaf block 1 in an index-organized table might contain entries as follows, ordered by primary key:. A scan of the index-organized table rows in primary key order reads the blocks in the following sequence:. To contrast data access in a heap-organized table to an index-organized table, suppose block 1 of a heap-organized departments table segment contains rows as follows:.
A B-tree index leaf block for this heap-organized table contains the following entries, where the first value is the primary key and the second is the rowid :. A scan of the table rows in primary key order reads the table segment blocks in the following sequence:. When creating an index-organized table, you can specify a separate segment as a row overflow area.
In index-organized tables, B-tree index entries can be large because they contain an entire row, so a separate segment to contain the entries is useful. In contrast, B-tree entries are usually small because they consist of the key and rowid. If a row overflow area is specified, then the database can divide a row in an index-organized table into the following parts:. This part contains column values for all the primary key columns, a physical rowid that points to the overflow part of the row, and optionally a few of the non-key columns.
This part is stored in the index segment. This part contains column values for the remaining non-key columns. This part is stored in the overflow storage area segment. A secondary index is an index on an index-organized table. In a sense, it is an index on an index. The secondary index is an independent schema object and is stored separately from the index-organized table. As explained in "Rowid Data Types" , Oracle Database uses row identifiers called logical rowids for index-organized tables.
A logical rowid is a baseencoded representation of the table primary key. The logical rowid length depends on the primary key length. Rows in index leaf blocks can move within or between blocks because of insertions. Rows in index-organized tables do not migrate as heap-organized rows do see "Chained and Migrated Rows". Because rows in index-organized tables do not have permanent physical addresses, the database uses logical rowids based on primary key. For example, assume that the departments table is index-organized.
The table stores rows as follows, with the last value as the location ID:. Secondary indexes provide fast and efficient access to index-organized tables using columns that are neither the primary key nor a prefix of the primary key. For example, a query of the names of departments whose ID is greater than could use the secondary index to speed data access.
Oracle Database Administrator's Guide to learn how to create secondary indexes on an index-organized table. Secondary indexes use the logical rowids to locate table rows. A logical rowid includes a physical guess , which is the physical rowid of the index entry when it was first made. Oracle Database can use physical guesses to probe directly into the leaf block of the index-organized table, bypassing the primary key search.
When the physical location of a row changes, the logical rowid remains valid even if it contains a physical guess that is stale. For index-organized tables, access by a secondary index varies, depending on the use and accuracy of physical guesses:. Without physical guesses, access involves two index scans: a scan of the secondary index followed by a scan of the primary key index. A secondary index on an index-organized table can be a bitmap index.
As explained in "Bitmap Indexes" , a bitmap index stores a bitmap for each index key. When bitmap indexes exist on an index-organized table, all the bitmap indexes use a heap-organized mapping table. The mapping table stores the logical rowids of the index-organized table. Each mapping table row stores one logical rowid for the corresponding index-organized table row. The database accesses a bitmap index using a search key. If the database finds the key, then the bitmap entry is converted to a physical rowid.
With heap-organized tables, the database uses the physical rowid to access the base table. With index-organized tables, the database uses the physical rowid to access the mapping table, which in turn yields a logical rowid that the database uses to access the index-organized table.
This chapter contains the following sections: Overview of Indexes Overview of Index-Organized Tables Overview of Indexes An index is an optional structure, associated with a table or table cluster , that can sometimes speed data access. To speed access, the manager could create an index that sequentially lists every employee ID with its folder location: ID Box 3, position 1 bottom ID Box 7, position 8 ID Box 1, position 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.
See Also: Chapter 5, "Data Integrity". Index Characteristics Indexes are schema objects that are logically and physically independent of the data in the objects with which they are associated. Note: If you drop an index, then applications still work. However, access of previously indexed data can be slower.
Keys and Columns A key is a set of columns or expressions on which you can build an index. Note: Primary and unique keys automatically have indexes, but you might want to create an index on a foreign key. Composite Indexes A composite index , also called a concatenated index , is an index on multiple columns in a table. Note: In some cases, such as when the leading column has very low cardinality, the database may use a skip scan of this index see "Index Skip Scan". Unique and Nonunique Indexes Indexes can be unique or nonunique.
Types of Indexes Oracle Database provides several indexing schemes, which provide complementary performance functionality. The indexes can be categorized as follows: B-tree indexes These indexes are the standard index type.
B-tree indexes have the following subtypes: Index-organized tables An index-organized table differs from a heap-organized because the data is itself the index. Reverse key indexes In this type of index, the bytes of the index key are reversed, for example, is stored as Descending indexes This type of index stores data on a particular column or columns in descending order.
B-tree cluster indexes This type of index is used to index a table cluster key. Bitmap and bitmap join indexes In a bitmap index, an index entry uses a bitmap to point to multiple rows. Function-based indexes This type of index includes columns that are either transformed by a function, such as the UPPER function, or included in an expression.
Application domain indexes This type of index is created by a user for data in an application-specific domain. B-Tree Indexes B-trees, short for balanced trees , are the most common type of database index. Branch Blocks and Leaf Blocks A B-tree index has two types of blocks: branch blocks for searching and leaf blocks that store values. Note: Indexes in columns with character data are based on the binary values of the characters in the database character set.
Domain indexes are built using the indexing logic supplied by a user-defined index type. An index type provides an efficient mechanism to access data that satisfy certain operator predicates. Typically, the user-defined index type is part of an Oracle option, like the Spatial option. For example, the SpatialIndextype allows efficient search and retrieval of spatial data that overlap a given bounding box.
The cartridge determines the parameters you can specify in creating and maintaining the domain index. Similarly, the performance and storage characteristics of the domain index are presented in the specific cartridge documentation. A further advantage of this approach is that the old index is still available for queries while the rebuild is in progress.
These statistics can be used to determine if the index should be rebuild or not. Oracle can understand whether using an index will improve the performance in the given query.
If Oracle thinks using an index will improve performance, it will use the index otherwise it will ignore the index. The above query will use the index as we are trying to get information about an emp based on the name. The above query will not use the index as the where clause does not select the column which has an index.
Oracle used to lock the table on which index is being created throughout the creation process in older versions. This makes the table unavailable for data manipulation during the creation of the index. Basically, with an online rebuild, Oracle locks the table at the start and end of the creation of the index.
Number of instances across which the indexes to be scanned. Indicates whether the index is on a temporary table. Indicates whether the name of the index is system generated Y or not N. Name of the default buffer pool to be used for the index blocks.
For a domain index, the owner of the indextype. For a domain index, the name of the indextype. For a domain index, the parameter string.
For partitioned indexes, indicates whether statistics were collected by analyzing the index as a whole YES or were estimated from statistics on underlying index partitions and subpartitions NO. Performance would really be down the tubes. The right number of indexes per table is somewhere between 0 and infinity. It'll vary based on need. You never want to have more indexes then you really need -- but you want to have every index you do.
Rating 15 ratings Is this answer out of date? If it is, please let us know via a Comment Comments Comment But A reader, September 22, - pm UTC. You can also catch regular content via Connor's blog and Chris's blog.
Or if video is more your thing, check out Connor's latest video and Chris's latest video from their Youtube channels. And of course, keep up to date with AskTOM via the official twitter account. Questions Limit in the number of indexes.
Question and Answer. You Asked Hi Tom, Is it a real limit in the number of indexes that you can have in a table? Tom, My question is whether in any situation would indexes slow down the query or will it cause the optimizer to take more time in arriving at the best path?
Like say an index on a date field. There could be any number of records having the same date. This question is because we often query all records within a date range. Other situation is having multiple indexes on one field with different combinations. Would this impact the queries which use only id in the where clause? This is with respect to OLAP application. Thanks very much Tom Really appreciate your efforts.
September 23, - am UTC. Sure, absolutely, but that can happen with 1, 2, 3, 4, 5, If you use the CBO and analyze your objects, this becomes less of a concern as the optimizer will understand if the data is selective or not. In your case, you said you already did the analysis, all indexes are needed -- hence they must be selective or you wouldn't need them. Hi Tom, 1. Is it good to index all columns in a table if required?
Is there any alternative? I heared that date columns should not be indexed. Is it true? It used to be the case I was told that once the RBO had considered 5 indexes on a table for use in a query, it picked the best of those 5 and did not consider any others.
We had our database design checked by an Oracle consultant "guru" some years ago, and he advised dropping all but 5 indexes on our main table to "improve" performance, citing the reason above.
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