TYPO3 Core

Quoting from https://jira.neos.io/browse/FLOW-326:

QueryResult::count() always execute a COUNT query, even if the results have been fetched or counted already.
This is especially an issue because we make use of this method a lot under the hood.
For example this (simplified but realistic) Fluid snippet:

<f:if condition="{products}">
  <h1>{products -> f:count()} products:</h1>
  <f:widget.paginate objects="{products}" as="paginatedProducts">
    <ul>
      <f:for each="{paginatedProducts}" as="product" iteration="iteration">
        <li>{product.name}</li>
      </f:for>
    </ul>
  </f:widget.paginate>
</f:if>

will execute the same SELECT COUNT(DISTINCT ... query four times.
And those counts can be very costly especially with complex queries or many rows in a INNODB table.