Originally Posted by epubfan
Hm, then all informations in 1 db-column and searches will be faster or what is the trick?
You wrote you search in title, author, date, size, words, language, format
With the exception of format, these are database columns, the database is 'heavily indexed' in order to make searching and sorting fast.
However, 'formats' refers to the extensions of files in book folders, hence a search (or sort) must crawl through the library folder tree looking for files whose extensions match the search term, which is slow.
But, 'approximate_formats()' refers to a derived list of formats that you need to put in a custom database column which will be fast for the same reasons other database column based searches are.
See the link that Peter T posted for a discussion of why & how the function came to implemented. And see http://manual.calibre-ebook.com/temp...ximate-formats
for definition and caveats