debug_parallel_query

doc html

<p>
        Allows the use of parallel queries for testing purposes even in cases
        where no performance benefit is expected.
        The allowed values of <code class="varname">debug_parallel_query</code> are
        <code class="literal">off</code> (use parallel mode only when it is expected to improve
        performance), <code class="literal">on</code> (force parallel query for all queries
        for which it is thought to be safe), and <code class="literal">regress</code> (like
        <code class="literal">on</code>, but with additional behavior changes as explained
        below).
       </p><p>
        More specifically, setting this value to <code class="literal">on</code> will add
        a <code class="literal">Gather</code> node to the top of any query plan for which this
        appears to be safe, so that the query runs inside of a parallel worker.
        Even when a parallel worker is not available or cannot be used,
        operations such as starting a subtransaction that would be prohibited
        in a parallel query context will be prohibited unless the planner
        believes that this will cause the query to fail.  If failures or
        unexpected results occur when this option is set, some functions used
        by the query may need to be marked <code class="literal">PARALLEL UNSAFE</code>
        (or, possibly, <code class="literal">PARALLEL RESTRICTED</code>).
       </p><p>
        Setting this value to <code class="literal">regress</code> has all of the same effects
        as setting it to <code class="literal">on</code> plus some additional effects that are
        intended to facilitate automated regression testing.  Normally,
        messages from a parallel worker include a context line indicating that,
        but a setting of <code class="literal">regress</code> suppresses this line so that the
        output is the same as in non-parallel execution.  Also,
        the <code class="literal">Gather</code> nodes added to plans by this setting are hidden
        in <code class="literal">EXPLAIN</code> output so that the output matches what
        would be obtained if this setting were turned <code class="literal">off</code>.
       </p>

documentation revision

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