Databases
StackOverflow2013
Postsdbo
POHow to make these two simultaneous transactions succeed at all times, without causing a uniqueness violation?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POHow to make these two simultaneous transactions succeed at all times, without causing a uniqueness violation?
    primarykey
    data
    text
    <p>Let's pretend that there's a <code>tags</code> table with a unique field called <code>name</code>.</p> <p>I have a transaction in which I perform a select to see if a tag with a specific name exists, and if it wasn't I create it:</p> <pre><code>START TRANSACTION; SELECT * FROM TAGS WHERE NAME = "FOO"; -- IF A TAG NAMED "FOO" DIDN'T EXIST THEN INSERT INTO TAGS VALUES("FOO"); COMMIT; </code></pre> <p>When two clients run this transaction in the default isolation level (repeatable read), this interleaving will lead one of them to fail with a uniqueness violation:</p> <pre><code>START TRANSACTION; START TRANSACTION; SELECT * FROM TAGS WHERE NAME = "FOO"; SELECT * FROM TAGS WHERE NAME = "FOO"; -- IF A TAG NAMED "FOO" DIDN'T EXIST THEN INSERT INTO TAGS VALUES("FOO"); -- IF A TAG NAMED "FOO" DIDN'T EXIST THEN INSERT INTO TAGS VALUES("FOO"); COMMIT; COMMIT; </code></pre> <p>I thought if I set the isolation level to serializable, I can avoid this situation, but I noticed that the same interleaving will then lead to a deadlock.</p> <p>How can I modify the transaction so that it never fails due to uniqueness constraint violation?</p> <p>For the record, this is the Ruby on Rails (ActiveRecord) code that corresponds to this scenario:</p> <pre><code>class Tag &lt; ActiveRecord::Base def self.create_tag(name) transaction do # setting isolation level to serializable leads to a deadlock # Tag.connection.execute("SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE") gets tag = Tag.find_by_name(name) if tag.nil? gets Tag.create!(:name =&gt; name) end end end end </code></pre>
    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. USK Everest
    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
    1. VO
      singulars
      1. POHow to make these two simultaneous transactions succeed at all times, without causing a uniqueness violation?
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POHow to make these two simultaneous transactions succeed at all times, without causing a uniqueness violation?
      1. USTimo Saloranta
      1. VTFavorite
    3. VO
      singulars
      1. POHow to make these two simultaneous transactions succeed at all times, without causing a uniqueness violation?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COkind of offtopic, but better use select name from tags where name = 'foo'; this will be index-covered query, which is faster than accessing the table
      singulars
      1. POHow to make these two simultaneous transactions succeed at all times, without causing a uniqueness violation?
      1. USMaxim Krizhanovsky
    2. COCan you performs Insert ... select? That [use mutex table](http://www.xaprb.com/blog/2005/09/25/insert-if-not-exists-queries-in-mysql/)
      singulars
      1. POHow to make these two simultaneous transactions succeed at all times, without causing a uniqueness violation?
      1. USMaxim Krizhanovsky
 

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