Reduce DB load

src/main/kotlin/mdnet/cache/ImageStorage.kt

@@ -130,28 +130,30 @@ class ImageStorage(
         LOGGER.info { "Cache at $size out of $maxSize bytes" }
         // we need to prune the cache now
         if (size > maxSize * 0.95) {
-            val toClear = size - (maxSize * 0.9).toLong()
+            var toClear = size - (maxSize * 0.9).toLong()
             LOGGER.info { "Evicting at least $toClear bytes from cache" }
 
-            val list = database.useConnection { conn ->
-                conn.prepareStatement(IMAGES_TO_PRUNE).apply {
-                    setLong(1, toClear)
-                }.use { stmt ->
-                    stmt.executeQuery().let {
-                        val ret = ArrayList<String>()
+            while (toClear > 0) {
+                val list = database.useConnection { conn ->
+                    conn.prepareStatement(IMAGES_TO_PRUNE).use { stmt ->
+                        stmt.executeQuery().let {
+                            val ret = ArrayList<Pair<String, Int>>()
 
-                        while (it.next()) {
-                            ret.add(it.getString(1))
+                            while (it.next()) {
+                                ret.add(it.getString(1) to it.getInt(2))
+                            }
+
+                            ret
                         }
-
-                        ret
                     }
                 }
-            }
 
-            for (id in list) {
-                LOGGER.info { "Evicting images $id from cache" }
-                deleteImage(id)
+                val count = list.fold(0) { sum, (id, num) ->
+                    deleteImage(id)
+                    sum + num
+                }
+                LOGGER.info { "Evicting $count bytes from cache this loop" }
+                toClear -= count
             }
         }
     }

src/main/kotlin/mdnet/cache/metadata.kt

@@ -34,16 +34,12 @@ create table if not exists Images(
     accessed timestamp not null default CURRENT_TIMESTAMP,
     disk_size integer as ((size + 4095) / 4096 * 4096)
 );
-create index if not exists Images_lastAccessed_idx on Images(accessed, disk_size, id);
+drop index if exists Images_lastAccessed_idx;
+create index if not exists Images_accessed on Images(accessed);
 """
 
 const val SIZE_TAKEN_SQL = "select sum(disk_size) from Images"
 
 const val IMAGES_TO_PRUNE = """
-select id from (
-    select id, sum(disk_size)
-        OVER (order by accessed rows unbounded preceding exclude current row)
-        as RunningTotal from Images
-) as X
-WHERE coalesce(X.RunningTotal, 0) <= ?;
+select id, disk_size from Images order by accessed asc limit 1000
 """