Databases
StackOverflow2013
Postsdbo
POPerformance Penalties for Unused Joins
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POPerformance Penalties for Unused Joins
    primarykey
    data
    text
    <p>I'm writing a script that generates a report based on a query that uses several tables joined together. One of the inputs to the script is going to be a list of the fields that are required on the report. Depending on the fields requested, some of the tables might not be needed. My question is: is there a [significant] performance penalty for including a join when if it is not referenced in a SELECT or WHERE clause?</p> <p>Consider the following tables:</p> <pre><code>mysql&gt; SELECT * FROM `Books`; +----------------------+----------+ | title | authorId | +----------------------+----------+ | Animal Farm | 3 | | Brave New World | 2 | | Fahrenheit 451 | 1 | | Nineteen Eighty-Four | 3 | +----------------------+----------+ mysql&gt; SELECT * FROM `Authors`; +----+----------+-----------+ | id | lastName | firstName | +----+----------+-----------+ | 1 | Bradbury | Ray | | 2 | Huxley | Aldous | | 3 | Orwell | George | +----+----------+-----------+ </code></pre> <p>Does</p> <pre><code>SELECT `Authors`.`lastName` FROM `Authors` WHERE `Authors`.`id` = 1 </code></pre> <p>Outperform:</p> <pre><code>SELECT `Authors`.`lastName` FROM `Authors` JOIN `Books` ON `Authors`.`id` = `Books`.`authorId` WHERE `Authors`.`id` = 1 </code></pre> <p>?</p> <p>It seems to me that MySQL should just know to ignore the JOIN completely, since the table is not referenced in the SELECT or WHERE clause. But somehow I doubt this is the case. Of course, this is a really basic example. The actual data involved will be much more complex.</p> <p>And really, it's not a terribly huge deal... I just need to know if my script needs to be "smart" about the joins, and only include them if the fields requested will rely on them.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. USThomas Kelley
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POPerformance Penalties for Unused Joins
      1. USwebdad3
      1. VTFavorite
    2. VO
      singulars
      1. POPerformance Penalties for Unused Joins
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POPerformance Penalties for Unused Joins
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThe cases that you have listed really can return different results. When you are joining the tables the returned results will not be all authors, they will be all authors who have written a book.
      singulars
      1. POPerformance Penalties for Unused Joins
      1. USJohn Kane
    2. COOh, good point... What if it's a left join? Then it's completely unnecessary and returns the same results. In that case, I wonder if there would be a performance benefit to leaving out the join?
      singulars
      1. POPerformance Penalties for Unused Joins
      1. USThomas Kelley
    3. COOh, but even with a left join, there would be a potential for duplicates (in this case, if the WHERE clause was: `Authors`.`id` = 3). So MySQL will always need to see what's in that table first... So it's always going to be at least a little bit of a drop in performance. Got it. Thanks!
      singulars
      1. POPerformance Penalties for Unused Joins
      1. USThomas Kelley
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload