Optimize search3, by removing OFFSET when paginating (#2655)

* Optimize pagination, removing offset

* For search, don't add `where` clause for empty queries

* Revert "Replace `COUNT(DISTINCT primary_key)` with `COUNT(*)`"

Genres are required as part of the count queries, so filter by genres work

* Optimize search3 query, using order by id if it is a "" query.

Also fix the optimizePagination query logic

* Allow offset optimizer threshold to be configured

conf/configuration.go

@@ -96,6 +96,7 @@ type configOptions struct {
 	DevSidebarPlaylists              bool
 	DevEnableBufferedScrobble        bool
 	DevShowArtistPage                bool
+	DevOffsetOptimize                int
 	DevArtworkMaxRequests            int
 	DevArtworkThrottleBacklogLimit   int
 	DevArtworkThrottleBacklogTimeout time.Duration
@@ -352,6 +353,7 @@ func init() {
 	viper.SetDefault("devenablebufferedscrobble", true)
 	viper.SetDefault("devsidebarplaylists", true)
 	viper.SetDefault("devshowartistpage", true)
+	viper.SetDefault("devoffsetoptimize", 50000)
 	viper.SetDefault("devartworkmaxrequests", number.Max(2, runtime.NumCPU()/3))
 	viper.SetDefault("devartworkthrottlebackloglimit", consts.RequestThrottleBacklogLimit)
 	viper.SetDefault("devartworkthrottlebacklogtimeout", consts.RequestThrottleBacklogTimeout)