Optimize thread search using matched docid sets.

This reduces thread search's 1+2t Xapian queries (where t is the
number of matched threads) to 1+t queries and constructs exactly one
notmuch_message_t for each message instead of 2 to 3.
notmuch_query_search_threads eagerly fetches the docids of all
messages matching the user query instead of lazily constructing
message objects and fetching thread ID's from term lists.
_notmuch_thread_create takes a seed docid and the set of all matched
docids and uses a single Xapian query to expand this docid to its
containing thread, using the matched docid set to determine which
messages in the thread match the user query instead of using a second
Xapian query.

This reduces the amount of time required to load my inbox from 4.523
seconds to 3.025 seconds (1.5X faster).

1 files changed, 15 insertions, 2 deletions

diff --git a/lib/notmuch-private.h b/lib/notmuch-private.h
index 592cfb2b..303aeb30 100644
--- a/lib/notmuch-private.h
+++ b/lib/notmuch-private.h
@@ -156,6 +156,8 @@ typedef enum _notmuch_private_status {
      :									\
      (notmuch_status_t) private_status)
 
+typedef struct _notmuch_doc_id_set notmuch_doc_id_set_t;
+
 /* database.cc */
 
 /* Lookup a prefix value by name.
@@ -222,8 +224,8 @@ _notmuch_directory_get_document_id (notmuch_directory_t *directory);
 notmuch_thread_t *
 _notmuch_thread_create (void *ctx,
 			notmuch_database_t *notmuch,
-			const char *thread_id,
-			const char *query_string,
+			unsigned int seed_doc_id,
+			notmuch_doc_id_set_t *match_set,
 			notmuch_sort_t sort);
 
 /* message.cc */
@@ -239,6 +241,9 @@ _notmuch_message_create_for_message_id (notmuch_database_t *notmuch,
 					const char *message_id,
 					notmuch_private_status_t *status);
 
+unsigned int
+_notmuch_message_get_doc_id (notmuch_message_t *message);
+
 const char *
 _notmuch_message_get_in_reply_to (notmuch_message_t *message);
 
@@ -426,6 +431,14 @@ _notmuch_mset_messages_get (notmuch_messages_t *messages);
 void
 _notmuch_mset_messages_move_to_next (notmuch_messages_t *messages);
 
+notmuch_bool_t
+_notmuch_doc_id_set_contains (notmuch_doc_id_set_t *doc_ids,
+                              unsigned int doc_id);
+
+void
+_notmuch_doc_id_set_remove (notmuch_doc_id_set_t *doc_ids,
+                            unsigned int doc_id);
+
 /* message.cc */
 
 void