Bug #79211 [Opn->Nab]: DomXPath::query() has very inconsistent performance

Edit report at https://bugs.php.net/bug.php?id=79211&edit=1 ID: 79211 Updated by: cmb@php.net Reported by: tom at r dot je Summary: DomXPath::query() has very inconsistent performance -Status: Open +Status: Not a bug Type: Bug Package: DOM XML related Operating System: Linux PHP Version: 7.4.2 -Assigned To: +Assigned To: cmb Block user comment: N Private report: N New Comment: I get rather different results with libxml 2.9.10 on PHP-7.4: 0.59029412269592 0.31803107261658 0.0068929195404053 This is not really unexpected, but isn't a PHP issue, anyway, since the performance difference would actually be caused by the libxml2 function xmlXPathEvalExpression(). Feel free to report that issue upstream. Previous Comments: ------------------------------------------------------------------------ [2020-02-02 04:35:24] tom at r dot je Description: ------------ Here are three sample queries Equivalent of the CSS tr td.one //tr//td[contains(concat(\' \', normalize-space(@class), \' \'), \' one \')] Equivalent of the CSS td.one //td[contains(concat(\' \', normalize-space(@class), \' \'), \' one \')] Equivalent of the CSS tr td //tr//td There's a test script below that uses these queries on a 1000x6 HTML table. I'd expect them all to take roughly the same amount of time to run. Test script: --------------- <?php $xml = '<table>'; for ($i = 0; $i < 1000; $i++) { $xml .= '<tr><td class="one">A</td><td class="two">B</td><td class="three">C</td><td class="four">D</td><td class="five">E</td><td class="six">F</td> </tr>'; } $xml .= '</table>'; $doc = new \DomDocument; $doc->loadXml($xml); $xpath = new \DomXpath($doc); $t1 = microtime(true); // tr td.one $xpath->query('//tr//td[contains(concat(\' \', normalize-space(@class), \' \'), \' one \')]'); $t2 = microtime(true); echo '<p>tr td.one: ' . ($t2 - $t1) . '</p>'; $t1 = microtime(true); // td.one $xpath->query('//td[contains(concat(\' \', normalize-space(@class), \' \'), \' one \')]'); $t2 = microtime(true); echo '<p>td.one: ' . ($t2 - $t1) . '</p>'; $t1 = microtime(true); // tr td $xpath->query('//tr//td'); $t2 = microtime(true); echo '<p>tr td: ' . ($t2 - $t1) . '</p>'; Expected result: ---------------- While I'd expect some variation in speed, the first expression seems unreasonably slow in comparison to the others. Actual result: -------------- tr td.one: 0.26365518569946 td.one: 0.0054380893707275 tr td: 0.00074887275695801 Finding all td elements by class name is fast Finding all td elements inside a td is fast Combining those, finding all td elements by class inside a tr is 50 times slower than just finding all td elements by class. Since the first expression is a combination of the other two, how can it be 50 times slower? It's clearly not the concat and normalize-space functions that are slowing it down because the second expression is fine. ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=79211&edit=1