Google’s Gary Illyes confirmed that Google does indeed check to see if an XML sitemap has a change in it before reprocessing the XML sitemap file.
The reason Google does this is because it doesn’t want to waste resources parsing and processing a file that is not changed from the last time that file was parsed and processed.
This question came up at the 5:33 mark in the SEO office hours.
Question: Does Google do any kind of comparisons between current and previous XML sitemap versions?
Answer: Well, the absolute answer is, yes!
We will not reprocess a sitemap that hasn’t changed since it was last crawled. But that’s just a software optimization to not waste resources, like computing resources.
As soon as you change something in your sitemap, be that the URL element or last mod, the sitemap will be parsed again and generally reprocessed. That doesn’t mean that the URLs will be surely crawled of course they are still subject to the quality evaluations like any other URL.
It’s also worth to mention that if you remove a URL from the sitemap because perhaps it doesn’t exist anymore that doesn’t mean it’s automatically going to be dropped from the index or even prioritized for crawling so it can be dropped sooner.
Here is the video embed:
The first point is somewhat obvious, I mean, it just makes computing sense.
That last point is not really new either but it is a good thing Gary mentioned it. Removing URLs from your XML sitemap won’t 100% result in that URL being removed.
Forum discussion at Twitter.