Since it is not possible to create the secondary INDEX for cluster tables, SAP came up with new concept called INDEX tables.

All the accounting documents (Open and closed) are stored in BKPF and BSEG tables and querying these tables takes lot of time. So SAP has created INDEX tables for BSEG which are updated with specific data based on classification and are much faster than BSEG.
 
EG: Customer documents, Vendor documents etc.
 
INDEX TABLES:
 
BSID- Accounting: Secondary Index for Customers (Open Items)
BSAD- Accounting: Secondary Index for Customers (Cleared Items)
BSIK- Accounting: Secondary Index for Vendors (Open Items)
BSAK- Accounting: Secondary Index for Vendors (Cleared Items)
BSIS- Accounting: Secondary Index for G/L Accounts (Open Items)
BSAS- Accounting: Secondary Index for G/L Accounts (Cleared Item)
 
How to remember these tables???
Remember BS*  *
 
3^rd letter indicates whether item is open or closed.
I - For open items.
A-    For closed items.
 
4^th letter indicates classification. 
     D- For customers
     K- For vendors
     S- For G/L accounts.
 
These tables are some examples which is mostly required in real time scenarios. There are few more INDEX tables like this.

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1. SAP Tables Part I
2. Improving Workflow Performance
3. Transport SAP Table contents

This reference guide focuses on following topics:

• ENTERPRISE RESOURCE PLANNING (ERP)
• SAP ADVANTAGES
• Role of ABAPer
• R/3 ARCHITECTURE COMPONENTS
• WHAT IS CLIENT IN SAP
• Developemt Obects -  REPOSITORY OBJECTS and  DATA DICTIONARY OBJECTS
• ABAP GUI
• DATA DICTIONARY
• INTERNAL TABLES
• SUBROUTINES
• Reports
• MODULE POOL PROGRAMMING
• Menu Painter
• BATCH DATA COMMUNICATION
• LSMW ( Legacy System Migration Workbench)
• SAPSCRIPTS
• SMARTFORMS

Link : Complete Reference to ABAP Basics

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3. Lock Objects in SAP

Let me tell you the steps.

1. Go to SQ02 transaction (Infoset: Initial Screen).
2. Select or Highlight the Infoset corresponding to the query.
3. Go to –> Query Directory –> (you will see the infoset in the name being displayed) Execute.
4. Select the Query you would want to unlock.
5. Click on Cancel Edit lock button or Edit –> Cancel edit lock or Shift + F6
Done…!!!! Query is unlocked.

You might also be interested in these posts:

1. Restricting SAP Queries from execution
2. SAP Statistical Query
3. Create a TCODE for an ABAP Query

SAP query provides the following type of reports:

• Basic List        Simple report  
• Statistics          Report containing statistical functions like Average &Percentage  
• Ranked List     Report written for analytical purpose

The following PDF shows the steps to create Statistical Query.

Link: SAP Statistical Query

You might also be interested in these posts:

1. Unlocking SAP Query
2. Create a TCODE for an ABAP Query
3. Restricting SAP Queries from execution

The field ‘authorization group’ allows you to enter an authorization group up to eight characters long. If you specify an authorization group, it is inherited as an attribute by every report generated for a query via this InfoSet. This means that only users authorized in their user master records to execute programs from this authorization group can execute these queries. Authorization groups thus provide a way of protecting reports generated by the query from unauthorized execution.

In SQ02: Change InfoSet > Goto Global Properties > Enter in Authorization group
In SQ01: Query > More functions > Generate program

You might also be interested in these posts:

1. Unlocking SAP Query
2. Assign/Create Authorization Group for a Table in SAP
3. Create a TCODE for an ABAP Query