Running WordPress.com means having multimillion-record database tables. Tables which we often need to batch-query.
Provided we could hardly select (or update, etc) millions of records at once and expect speed, we commonly have to “page” our scripts to only handle a limited number of records at once, then move on to the next batch.
Classic, but inefficient, solution
The usual way of paging result sets in most SQL RDMS is to use the
OFFSET option (or
LIMIT [offset], [limit], which is the same).
But on a performance level, this means you’re asking your DB engine to figure out where to start from all on its own, every time. Which then means it must be aware of every record before the queried offset, because they could be different between queries (deletes, etc). So the higher your offset number, the longer the overall query will take.
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