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.

Oracle Bitmap Index

Common usage of Bitmap Indexes is a data warehousing environment. This implies large amounts of data, high level of ad-hoc queries but a low level of concurrent DLM transactions.

Why to use?

  • reduced response time
  • reduced storage requirements compared to other indexing techniques
  • efficient maintenance during parallel DML and load

Bitmap indexes are typically only a fraction of the size of the indexed data in the table.

How it works?

An index provides pointers to the rows in a table that contain a given key value. A regular index stores a list of rowids for each key corresponding to the rows with that key value. In a bitmap index, a bitmap for each key value replaces a list of rowids.

Each bit in the bitmap corresponds to a possible rowid, and if the bit is set, it means that the row with the corresponding rowid contains the key value. A mapping function converts the bit position to an actual rowid, so that the bitmap index provides the same functionality as a regular index. Bitmap indexes store the bitmaps in a compressed way. If the number of distinct key values is small, bitmap indexes compress better and the space saving benefit compared to a B-tree index becomes even better.

Best use case?

From my opinion, bitmaps are most effective on queries with multiple where clauses.  As the bitmaps from bitmap indexes can be combined quickly, it is usually best to use single-column bitmap indexes. This is why the DW environment is the “home” for this type of indexing.

Frequent use cases?

The advantage of bitmap indexes is higher on columns where degree of cardinality (number of distinct values for the indexed column versus the total number of rows in the table) is small. Columns like Status, gender are optimal examples.

However, the datawarehouse environments can also benefit from bitmaps on columns with higher cardinality. This is also mostly due to the combining of bitmaps for quick filtering.

This is due to the fact that AND and OR conditions in the where clause can we resolved faster by performing Boolean operations directly on the bitmaps before converting them to rowids for data retrieval.

Using this bitmap merge methodology, Oracle can provide sub-second response time when working against multiple low-cardinality columns.

Unlike most other indexing, bitmap indexes include rows that have the null values.

Bitmap and Partitioning

You can create bitmap indexes on partitioned tables but they must be local to the partitioned table—they cannot be global indexes.

Example

create table test_tb
( row_id number
, text varchar2(100)
, status char(6));

create bitmap index test_bmpidx on test_tb(status);

On partitioned tables:

create table 
 t2
(c1 char(3) not null
, c2 date not null
, c3 number
, c4 varchar2(100))
partition by range(c2)
interval (numtodsinterval (1,'day'))
 (
 partition empty values less than (to_Date ('03-OCT-2016', 'dd-mon-yyyy'))
 )
;

create bitmap index test_bmpidx on test_tb(status);

 

Restrictions

You cannot create a bitmap join index on a temporary table.

 

Cursor Loop Updates

I’m writing this topic mostly for database developer coming from the programming world.

I have seen various procedural units which use the for syntax to run updates on base tables. Scenarios like the one bellow.

FOR update SQL:

for i in (select * from t1) loop
 update t1
 set amount=amount*(1+(Select adjustment from t2 where discount=i.discount 
    and categ=i.categ))
 where row_id=i.row_id;
end loop;
commit;

The scenario above is the basic example where one table is updated using values from a second table.

Now, in order to understand the basic problem with the above example there is something else you need to keep in mind. In Oracle, the PL/SQL and the SQL are 2 very specific modules. Whenever you call a SQL statement in a PL/SQL block, you are doing a context switch. While this can be very easy on a simple statement, larger data volumes updates like the one above will show you the cost of that context switch.

The scope of this post is to create a thinking methodology shift into a database oriented one, explaining the pro an cons of this approach, together with options of conversion for this type of code into classical DML statements.

Basically, what I’m trying to suggest is use PL/SQL only when procedural thinking is required, but do use your SQL in the most efficient manner to avoid these costly mistakes. Note that 70-80% of these type of syntax use cases can be rewritten in plain SQL.

For instance, our prior for loop can be rewritten into :

merge into t1
 using t2
on ( t1.discount=t2.discount and t1.categ=t2.categ)
when matched then 
 update
 set amount=amount*(1+t2.adjustment);
commit;

Now,

The first example, using the cursor based update, in my demo VM test environment run for about 15 minutes to update a table of 100K rows using a mapping table of 19 rows

while the second occurrence took only  less than 2 seconds.

loop-updatemerge

Now please note this is a basic example, but I have not, as of now, met a situation where this was not the case.

Please note merge will not allow you to update the match condition key fields, therefore some situations might require you to adjust your syntax accordingly.

Scripts:

test-scripts

Please note scripts are uploaded as PDFs but you can still copy the code in.

Note: this was run on Oracle Database 12C 12.1.0.1.0