by Markus Winand.

Modifying Data


So far we have only discussed query performance, but SQL is not only about queries. It supports data manipulation as well. The respective commands—insert, delete, and update—form the so-called “data manipulation language” (DML)—a section of the SQL standard. The performance of these commands is for the most part negatively influenced by indexes.

An index is pure redundancy. It contains only data that is also stored in the table. During write operations, the database must keep those redundancies consistent. Specifically, it means that insert, delete and update not only affect the table but also the indexes that hold a copy of the affected data.

Contents

  1. Insert — cannot take direct benefit from indexes

  2. Delete — uses indexes for the where clause

  3. Update — does not affect all indexes of the table

Previous pageNext page

You can’t learn everything in one day. Subscribe the newsletter via E-Mail, Twitter or RSS to gradually catch up. Have a look at modern-⁠sql.com as well.

About the Author

Photo of Markus Winand

Markus Winand provides insights into SQL and shows how different systems support it at modern-sql.com. Previously he made use-the-index-luke.com, which is still actively maintained. Markus can be hired as trainer, speaker and consultant via winand.at.

Buy the Book

Cover of “SQL Performance Explained”: Squirrel running on grass

The essence of SQL tuning in 200 pages

Buy now!
(paperback and/or PDF)

Paperback also available at Amazon.com.

Hire Markus

Markus offers SQL training and consulting for developers working at companies of all sizes.
Learn more »

Connect with Markus Winand

Markus Winand on LinkedInMarkus Winand on XINGMarkus Winand on Twitter
“Use The Index, Luke!” by Markus Winand is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 Unported License.
Legal | Contact | NO WARRANTY | Trademarks | Privacy and GDPR | CC-BY-NC-ND 3.0 license