SQL Index Management: CREATE INDEX and DROP INDEX

Database indexes are special lookup tables that the database uses to speed up data retrieval. They don't store the actual data, but rather pointers to where the data is located, making searches significantly faster. However, remember that indexes impact the performance of data modifications (inserts, updates, deletes).

CREATE INDEX

Creating an Index

The CREATE INDEX command adds an index to a table. Indexes allow duplicate values in the indexed column(s).

CREATE INDEX index_name
ON table_name (column1, column2, ...);
      

index_name is the name you give to the new index. table_name is the name of the table you're adding the index to. You can index multiple columns by listing them separated by commas. The syntax for creating indexes can vary slightly between database systems (MySQL, PostgreSQL, SQL Server, etc.), so always check your database's documentation for the correct syntax.

Example: Creating a Single-Column Index

This example creates an index named "idx_lastname" on the "LastName" column of the "Persons" table.

CREATE INDEX idx_lastname ON Persons (LastName);
      

Example: Creating a Multi-Column Index

This creates an index on multiple columns ("LastName" and "FirstName").

CREATE INDEX idx_pname ON Persons (LastName, FirstName);
      

Important Note on Index Performance

While indexes speed up queries, they also slow down data modifications (inserts, updates, and deletes) because the index must be updated whenever data changes. Only index columns frequently used in WHERE clauses to optimize performance.

DROP INDEX

Dropping an Index

The DROP INDEX command removes an index from a table. This is useful if an index is no longer needed or is causing performance issues.

The syntax varies slightly depending on the database system:

DROP INDEX index_name ON table_name;
      

DROP INDEX table_name.index_name;
      

DROP INDEX index_name;
      

ALTER TABLE table_name DROP INDEX index_name;