Month: May 2020

Oracle Indexing

Indexing is one of the most frequent approaches when resolving query performance issues raised within a database (though not necessary the right approach, but we can pick this up later).

However, in order to better use indexing strategies, we should go trough the process on understanding index types and their functionality.

First of all, please keep in mind that an index is logically and physically independent of the data they represent. This implies that modifying the index will not affect the data consistency within the table the index is associated with.

It can be created on one or more columns of a table to enable queries to retrieve a small set of randomly distributed rows while reducing the cost of that operation, by reducing the IO associated with the alternative full table scan.

The general considerations for creating an index would be

  • Unique indexes for candidate unique/pk columns, to enable naming the index when creating the associate constraint on the table;
  • a referential constraint column
  • columns used in frequent queries with high selectivity (columns on which the filters applied would enable the return of a small percentage of the rows in the table).

! Note: Primary and unique keys automatically have indexes, but you might want to create an index on a foreign key.

Indexes are automatically maintained by the database with no additional action required by the user. This however, does not imply that an index comes without costs. Always, indexes will improve query performance, but decrease performance on data manipulations. That is due to the fact that any insert/update/delete will have to maintain both objects: the table the DML is submitted on as well as the index update.

 

When testing an indexing strategy, a developer can take advantage of the following properties of the indexes:

  • usability
  • visibility

Usability: Indexes are by default usable. An unusable index will both not be maintained by the DML operations, nor will it be used by the optimizer. This property can help improve performance on bulk loads. Instead of dropping and recreating the index, we can easily make it unusable and then rebuild.

! Note: Unusable indexes and index partitions do not consume space. When you make a usable index unusable, the database drops its index segment.

Syntax:

to set an index to unused:

alter index test_idx unusable;

unusable

to rebuild index:

alter index test_idx rebuild;

valid after rebuild

Visibility: Indexes are by default visible. An invisible index will still be maintained by DML operations but will not be used by the optimizer. Invisible indexes are especially useful for testing the removal of an index before dropping it or using indexes temporarily without affecting the overall application.

alter index test_idx invisible;

invisible

alter index test_idx visible;

to restore the index.

Index types (based on column number):

  • single key index
  • composite index

Index types (based on data content):

  • unique
  • nonunique

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).

!Note: 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.

Index types (based on structure of the index):

  • B-tree (balanced tree index) (standard type)
    • Index Organized Table (IOT)
    • Reverse key index
    • Descending index
    • B-tree cluster index
  • bitmap and bitmap join index
  • function based index

 

B-tree:

  • excellent for PK and highly-selective indexing
  • data retrieved sorted by the indexed columns

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.

IOT – this table differs from classical (heap-organized) table by the fact the data is in the index itself.

For more details on IOTs, please see related article here.

B-tree cluster indexes – 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.

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.

More on bitmap indexes here.

Function based index: 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. Example of function base b-tree here.

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Oracle Index Organized Table(IOT)

What is an IOT?

An IOT implies having your entire table indexed, within the same B-Tree structure. Should be used for larger scale tables or dimensions, highly used within the database.

How to?

Accessing the data is done via the primary key, much faster as the key and the data reside in the same structure. Also, lack of duplicaiton on key columns imply the total storage requirements are reduced.

To create an index organized table you must:

  • Specify the primary key using a column or table constraint.
  • Use the ORGANIZATION INDEX.

In addition you can use INCLUDING to define which non-key columns are stored with the key columns in the head piece, should overflow be necessary.

Example:

CREATE TABLE test_iot
 (id NUMBER(10)
, description VARCHAR2(50) NOT NULL
, comments varchar2(4000) 
, CONSTRAINT pk_test_tb PRIMARY KEY (id) ) 
ORGANIZATION INDEX 
TABLESPACE iot_tablespace 
INCLUDING description 
OVERFLOW TABLESPACE overflow_tablespace;

What to keep in mind when using IOT

Having a B-Tree index behaviour, the IOTs can become fragmented, or there might be other reasons you will need to rebuild the index. With no overflow it can be rebuild offline or online, while when it has overflow it can only be rebuit offline.

ALTER TABLE test_iot MOVE INITRANS 10;
--online rebuild
ALTER TABLE test_iot MOVE ONLINE INITRANS 10;
--offline with OVERFLOW
ALTER TABLE test_iot MOVE TABLESPACE iot_tablespace OVERFLOW TABLESPACE overflow_tablespace;

Oracle Function Based Index

What is a Function-Based Index?

A function-based index is an index created on the result of a function or expression.

This type of index is more used for situations with atypical searches on the specified columns (e.g. search on an user email address ignoring sensitivity can be optimised using a function based index; rounded up amounts.)

Frequent use cases?

This type of index is not behaving like our documented bitmap index, therefore the use cases will actually differ a lot. It can easily be used in cases of high cardinality distinct values, where the searches follow a particular generic pattern, but the data input can vary to a high degree.

Example

Consider the following table as a subscription table for email notifications on your particular site.

create table test_tb
( row_id number
, email_address varchar2(100)
, status char(6)
, gender varchar2(20)
, age number
, education varchar2(100)
, user_preferences varchar2(500)
);

create index test_bidx on test_tb(email_address);
create index test_fbidx on test_tb(upper(email_address));
create index test_fbidx_g on test_tb(upper(gender));

Now, if you are a marketing company wanting to analyse the data from muliple sites to asses users preferences for particular companies, you will want to be able to match the data for same user, so you can see the correlation between various data gathered. This is wherethe index will come in handy.

select count(*) as no_entries, upper(gender)
from test_tb
group by upper(gender);

Also, if you want to do an analysis based on email this can be very useful as a query like the one bellow will not use the test_bidx index, but the test_fbidx one:

select * from test_tb where upper(email_address) = 'no-reply@domain.com';