https://bugzilla.wikimedia.org/show_bug.cgi?id=52979

       Web browser: ---
            Bug ID: 52979
           Summary: ApiQueryAllUsers-related queries causing lag on
                    Wikidata
           Product: MediaWiki
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: Unprioritized
         Component: API
          Assignee: wikibugs-l@lists.wikimedia.org
          Reporter: b...@mzmcbride.com
                CC: bjor...@wikimedia.org, bryan.tongm...@gmail.com,
                    roan.katt...@gmail.com, s...@reedyboy.net
    Classification: Unclassified
   Mobile Platform: ---

We just had an issue where hung queries from the Web API were causing massive
lag on one of the Wikidata servers. Example query snippet:

---
SELECT /* ApiQueryAllUsers::execute */  ipb_deleted,COUNT(*) AS
recentedits,user_name
---

Lag as reported by
<https://www.wikidata.org/w/api.php?action=query&meta=siteinfo&siprop=dbrepllag&sishowalldb=>
was high:

---
<?xml version="1.0"?>
<api>
  <query>
    <dbrepllag>
      <db host="db1058" lag="0" />
      <db host="db1005" lag="0" />
      <db host="db1026" lag="26432" />
      <db host="db1021" lag="0" />
    </dbrepllag>
  </query>
</api>
---

Server log entry about killing the queries:
<https://wikitech.wikimedia.org/w/index.php?title=Server_Admin_Log&diff=80744&oldid=80742&diffonly=1>.

As soon as the queries were killed, lag decreased.

ApiQueryAllUsers needs investigation.

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