Single vs Concatenated Indexes

Terminology:

• Single Index = Single-Column Index
• Concatenated Index = Multi-Column Index

How indexes work:

• Index is used to find data fast, therefore it looks for the WHERE clause that defines the search condition

When to use Single-Column Indexes:

• When most of the time that indexed column is used in the WHERE clause

Index on (x): 

SELECT * FROM table WHERE x = 11

When to use Multi-Column Indexes:

• When more columns are used in the WHERE clause

Index on (t, x):

SELECT * FROM table WHERE t = 9 and x = 11