The update_document performs poorly if called multiple times

[nijel]

The IndexWriter.update_document method performs really poorly when invoked several times on index with an unique field. The reason is that it leads to constructing searcher for every updated document (what in turn leads to reading index from disk several times):

whoosh/src/whoosh/writing.py

Lines 480 to 489 in c823d16

	unique_fields = self._unique_fields(fields)
	if unique_fields:
	with self.searcher() as s:
	uniqueterms = [(name, fields[name]) for name in unique_fields]
	docs = s._find_unique(uniqueterms)
	for docnum in docs:
	self.delete_document(docnum)
	# Add the given fields
	self.add_document(**fields)

What should be considered is to create one searcher per writer (lazily once it is needed for the first time) and close it when writer is closed.

This can be currently workarounded by copying the update_document logic and constructing searcher outside. For example I've done this in WeblateOrg/weblate@91cb357