Five Tips to Get the Most Out of Your Indexing

Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Five Tips to Get the
Most out of Your Indexes
1
Maria Colgan
Master Product Manager
Oracle Database Server Technologies
June 2018
JEFF
@SQLMaria

B-Tree Indexes
– Available since 5, they are the
most common type of index
– Goal: minimize time to find
small amounts of data
– Including function-based,
Reverse Key Indexes, & IOTs
– One to one mapping, little
degradation in retrieval
performance as underlying
table grows
Bitmap Indexes
– Available since 7.3, designed
for read-mostly workloads
– Goal: Improve performance
of ad-hoc analytic queries
– A single bitmap key entry
points to many rows
– Multiple indexes can be used
together to determine rowids
– Bitmap Join indexes replaces
need for two table join
2
Text Search Indexes
– Available since 8i designed for
text retrieval
– Goal: Quickly identify large
coherent documents
– Can index documents of
different formats such as MS
Word, HTML or plain text
– A single entry points to many
documents
Oracle Offers a Variety of Different Indexing Techniques
Root Block
leaf Blocks
Branch Blocks

Reverse Key Indexes
• Reverse key indexes physically stores the bytes of the keys in reverse order
– CREATE INDEX sales_prod_reverse_idx ON sales(prod_id) REVERSE;
• Used to avoid “hot block syndrome” or buffer busy waits
– On columns with monotonically increasing values
– Eg: sequence populated fields or dates/timestamps
• At a cost
– Prevents some index range scans as 5 is not stored next to 6
• Equality predicates on non-unique index will use index range scan
• Range predicates such as between, <, > etc. may not use index
– Additional CPU evident mostly in single user mode, with multi-users you see less
overall CPU because you are not spinning on a buffer wait
Relieving Index contention

Partially Useable Indexes
• Enables the creation of local or global indexes on just a subset of partitions
• Indexing only the stable partitions (little or no DML) minimizes the impact
on ingest performance
• Queries accessing data only within the indexed partitions will use the index
• Queries that access data only in the non-indexed partitions will do full scan
• Queries accessing partitions with and without indexes can take advantage
of the query transformation called Table Expansion
• Supported by definition on Partition views
4

Partially Useable Indexes
CREATE TABLE sales(order_status char(6) not null, order_date…)
INDEXING ON
PARTITION BY LIST (order_status)(
PARTITION p1 VALUES(‘OPEN’) INDEXING OFF,
PARTITION p2 VALUES(‘CLOSED’));
5

Table Expansion Transformation
6
P1: ORDER_STATUS=‘OPEN’
SELECT * FROM sales
WHERE ORDER_DATE between‘2017-01-01’ and ‘2017-12-31’;
P2: ORDER_STATUS=‘CLOSED’
P1: Best access path is full table scan
SALES
P2: Best access path is index scan
Local index on
ORDER_DATE
IDX_SALES: ORDER_DATE

Table Expansion Transformation
----------------------------------------------------------------------
| Id | Operation | Name | Pstart| Pstop |
----------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | |
| 1 | SORT AGGREGATE | | | |
| 2 | VIEW | VW_TE_2 | | |
| 3 | UNION-ALL | | | |
| 4 | PARTITION RANGE SINGLE | | 1 | 1 |
|* 5 | TABLE ACCESS FULL | SALES | 1 | 1 |
| 6 | PARTITION RANGE SINGLE | | 2 | 2 |
|* 7 | INDEX RANGE SCAN | IDX_SALES | 2 | 2 |
----------------------------------------------------------------------
5 - access("ORDER_DATE“>=TO_DATE(' 2017-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND
"ORDER_DATE"<=TO_DATE(' 2018-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss'))
filter("ORDER_DATE“>=TO_DATE(' 2017-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND
7 - access("ORDER_DATE“>=TO_DATE(' 2017-01-01 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND

Indexing Nulls
• Bitmap indexes – always index nulls
• B*Tree cluster indexes – always index nulls
• B*Tree indexes do not if and only if the entire key is null (all columns)
8
CREATE INDEX i ON t(object_id,0);
SELECT * FROM t WHERE object_id is NULL;
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1445 | 149K| 96 (0)| 00:00:02 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1445 | 149K| 96 (0)| 00:00:02 |
|* 2 | INDEX RANGE SCAN | I | 1445 | | 7 (0)| 00:00:01 |
-------------------------------------------------------------------------------------

Copyright © 2017, Oracle and/or its affiliates. All rights reserved. | 9
• How do I know which indexes are actually
needed?
• New DBA_INDEX_USAGE views provide usage
histograms and access details for all indexes
• Track index usage with no overhead
Index Usage Statistics
Public

Index Usage Statistics
10Public
Index entries with no statistics
indicate indexes that currently are
not being used
These are potential candidates to
be deleted

INVISIBLE Indexes
• Ability to create and maintains an index that the optimizer is blind to
– Optimizer can’t use an invisible index as an access method
• Easy mechanism to begin the process of removing unused indexes
1. Mark the index invisible
ALTER INDEX i INVISIBLE;
2. If no one misses it for a complete business cycle, its safe to drop it
• Also useful to test out the benefit of a new index before publishing it
1. Create invisible index
CREATE INDEX i ON t(x, object_id) INVISIBLE;
2. Set parameter OPTIMIZER_USE_INVISIBLE_INDEXES to true in 1 session to test
ALTER SESSION SET optimizer_use_invisible_indexes=true;

Indexes are a critical tool to help improve performance
12
Indexes
ORACLE
Use them wisely to
greatly improve speed of
data access …..
…Over use them and you
will grind the system to a
halt!

Five Tips to Get the Most Out of Your Indexing

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