How do you create an SQL index in DB2?

You create an SQL index in DB2 for i (on IBM i) using the CREATE INDEX statement. Here's a breakdown of the syntax and options:

Basic Syntax :

CREATE INDEX index-name
ON table-name (column1, column2, ...);

index-name: The name you want to give to your index. It's good practice to use a naming convention that makes it clear what the index is for.
table-name: The name of the table on which you're creating the index.
column1, column2, ...: The columns that you want to include in the index. You can include one or more columns. The order of the columns is important, as it affects how the index is used.

Example :

CREATE INDEX CustNameIdx
ON CUSTOMERS (LastName, FirstName);

This creates an index named CustNameIdx on the CUSTOMERS table, using the LastName and FirstName columns. Queries that filter by LastName and then FirstName will be able to use this index efficiently.

Key Options and Considerations:

* ASC/DESC : You can specify whether you want the index to be in ascending (ASC) or descending (DESC) order for each column. Ascending is the default.

CREATE INDEX OrderDateIdx
ON ORDERS (OrderDate DESC);  -- Index in descending order of OrderDate?

* UNIQUE : You can create a unique index, which enforces that the combination of values in the indexed columns is unique across all rows in the table. This is often used for primary keys or other unique constraints.

CREATE UNIQUE INDEX CustNoIdx
ON CUSTOMERS (CustomerID);?

Partitioned Tables: If your table is partitioned, you might need to include partitioning columns in the index or specify how the index is partitioned.
Expression-based Indexes: You can create indexes on expressions involving columns, not just on the columns themselves. This can be useful for optimizing queries that filter or sort by calculated values.
```
CREATE INDEX UpperNameIdx
ON CUSTOMERS (UPPER(LastName));
```
Index Type: While DB2 for i typically uses B-tree indexes, there are other index types (like Encoded Vector Indexes) that might be appropriate for specific workloads or data characteristics. The default is usually a B-tree index.
Journaling: Indexes can be journaled, which is important for recovery in case of system failures. The journaling attributes of an index are often inherited from the table.
File vs. Index: It's important to distinguish between a file (which stores the actual data) and an index (which is a separate structure that helps speed up data access). An index is like the index in a book; it doesn't contain the data itself, but it tells you where to find it.

Best Practices:

Index selectively: Don't index every column. Indexes add overhead to data modification operations (inserts, updates, deletes). Only index the columns that are frequently used in WHERE clauses, JOIN conditions, ORDER BY clauses, or GROUP BY clauses.
Consider the order of columns: The order of columns in the index matters. Put the most frequently used columns first.
Test and monitor: After creating indexes, test your queries to make sure they're using the indexes effectively. You can use tools like Visual Explain to analyze query performance. Monitor your indexes over time to ensure they're still providing benefit.
Unique indexes for constraints: Use unique indexes to enforce primary key and unique constraints. This is often more efficient than using separate constraints.