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84 lines
4.0 KiB
Plaintext
Executable File
84 lines
4.0 KiB
Plaintext
Executable File
% layout 'default';
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% my $title = 'Help';
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% title $title;
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<h1><%= $title =%></h1>
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<p class="slogan"><strong>explain.depesz.com</strong> is a tool for finding real causes for slow queries.</p>
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<p>Generally, one would use the <code>EXPLAIN ANALYZE</code> query; and read the output. The problem is
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that not all parts of the output are easily understandable by anybody, and it's not always obvious
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whether a node that executes in 17.3ms is faster or slower than the one that runs in 100ms - given the
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fact that the first one is executed 7 times.</p>
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<p>To use the site, simply go to <a href="<%= url_for 'new-explain' %>" title="link to: new explain">first page</a>
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and paste there explain analyze output from your psql.</p>
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<p>After uploading, you will be directed to a page which shows parsed, and nicely (well, at least
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nice for me :) colorized output, to emphasize important parts.</p>
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<p>The url for colorized output is persistent, so you can easily show it to
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others. For example to those nice guys on the irc channel <em>#postgresql</em> on freenode.</p>
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<p>This graph uses 4 colors to mark important things:</p>
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<ul class="colors">
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<li class="c-1">white background - everything is fine</li>
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<li class="c-2">yellow background - given node is worrying</li>
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<li class="c-3">brown background - given node is more worrying</li>
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<li class="c-4">red background - given node is very worrying</li>
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</ul>
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<p>The color is chosen based on which mode you use: "<strong>Exclusive</strong>",
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"<strong>Inclusive</strong>" or "<strong>Rows X</strong>".</p>
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<h2>Exclusive</h2>
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<p>This is the total amount of time <a href="http://www.postgresql.org" title="link to PostgreSQL site">PostgreSQL</a> spent
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evaluating this node, without time spent in its subnodes. If the node has been executed many times (for example because
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of a <code>Nested Loop</code> plan), this time will be correctly multiplied.</p>
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<h3>Colors:</h3>
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<ul class="colors">
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<li class="c-1">white background - is chosen if exclusive time <= 10% of total query time</li>
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<li class="c-2">yellow background - is chosen if exclusive time ∈ (10%, 50%> of total query time</li>
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<li class="c-3">brown background - is chosen if exclusive time ∈ (50%, 90%> of total query time</li>
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<li class="c-4">red background - is chosen if exclusive time > 90% of total query time</li>
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</ul>
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<h2>Inclusive</h2>
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<p>This is just like <strong>Exclusive</strong>, but it doesn't exclude time of subnodes. So, by definition the top node will
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have <strong>Inclusive</strong> time equal to the total time of the query.</p>
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<h3>Colors:</h3>
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<ul class="colors">
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<li class="c-1">white background - is chosen if inclusive time <= 10% of total query time</li>
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<li class="c-2">yellow background - is chosen if inclusive time ∈ (10%, 50%> of total query time</li>
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<li class="c-3">brown background - is chosen if inclusive time ∈ (50%, 90%> of total query time</li>
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<li class="c-4">red background - is chosen if inclusive time > 90% of total query time</li>
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</ul>
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<h2>Rows X</h2>
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<p>This value stores information about how big the planner's mistake was when it estimated the return row count.</p>
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<p>For example, if planner estimated that a given node will return 230 rows, but it returned 14118 rows, the
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error is 14118/230 == 61.4.</p>
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<p>It has to be noted that if the numbers were the other way around (estimated 14118, but really only 230), the
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<strong>Rows X</strong> would be the same. To show whether planner underestimated or overestimated - there is
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an arrow showing either ↓ - if planner underestimated rowcount, or ↑ if it overestimated.</p>
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<h3>Colors:</h3>
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<ul class="colors">
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<li class="c-1">white background - is chosen if rows-x <= 10</li>
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<li class="c-2">yellow background - is chosen if rows-x ∈ (10, 100></li>
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<li class="c-3">brown background - is chosen if rows-x ∈ (100, 1000></li>
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<li class="c-4">red background - is chosen if rows-x > 1000</li>
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</ul>
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