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     <h2 class="title">
      <a name="performance-schema-restrictions">
      </a>
      29.20 Restrictions on Performance Schema
     </h2>
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  <p>
   The Performance Schema avoids using mutexes to collect or produce
      data, so there are no guarantees of consistency and results can
      sometimes be incorrect. Event values in
   <code class="literal">
    performance_schema
   </code>
   tables are nondeterministic
      and nonrepeatable.
  </p>
  <p>
   If you save event information in another table, you should not
      assume that the original events remain available later. For
      example, if you select events from a
   <code class="literal">
    performance_schema
   </code>
   table into a temporary
      table, intending to join that table with the original table later,
      there might be no matches.
  </p>
  <p>
   <a class="link" href="mysqldump.html" title="6.5.4 mysqldump — A Database Backup Program">
    <span class="command">
     <strong>
      mysqldump
     </strong>
    </span>
   </a>
   and
   <code class="literal">
    BACKUP
      DATABASE
   </code>
   ignore tables in the
   <code class="literal">
    performance_schema
   </code>
   database.
  </p>
  <p>
   Tables in the
   <code class="literal">
    performance_schema
   </code>
   database
      cannot be locked with
   <code class="literal">
    LOCK TABLES
   </code>
   , except the
   <code class="literal">
    setup_
    <em class="replaceable">
     <code>
      xxx
     </code>
    </em>
   </code>
   tables.
  </p>
  <p>
   Tables in the
   <code class="literal">
    performance_schema
   </code>
   database
      cannot be indexed.
  </p>
  <p>
   Tables in the
   <code class="literal">
    performance_schema
   </code>
   database are
      not replicated.
  </p>
  <p>
   The types of timers might vary per platform. The
   <a class="link" href="performance-schema-performance-timers-table.html" title="29.12.22.6 The performance_timers Table">
    <code class="literal">
     performance_timers
    </code>
   </a>
   table shows which
      event timers are available. If the values in this table for a
      given timer name are
   <code class="literal">
    NULL
   </code>
   , that timer is not
      supported on your platform.
  </p>
  <p>
   Instruments that apply to storage engines might not be implemented
      for all storage engines. Instrumentation of each third-party
      engine is the responsibility of the engine maintainer.
  </p>
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