After moving or removing media files on a Synology NAS the previews etc do not update correctly and will remain visible despite the files being gone. This script collected from the Synology forums removes the orphan data from the media database. Credits go to various users of the Synology forums.

remove_synology_media_orphans.sh

#!/bin/sh
# Usage: ./remove_orphans.sh [-f]
#
# Run this after moving / removing media files on a Synology NAS.
 
[ "$1" = "-f" ] && REMOVE=1
 
IFS='
'

DBADMIN=postgres

for db in directory; do
    for testfile in `/usr/bin/psql mediaserver $DBADMIN -tA -c "select path from $db;"`; do
        if [ ! -d "$testfile" ]; then
            echo "MISSING FOLDER: $testfile"
            [ -n "$REMOVE" ] && synoindex -D "$testfile"
        fi
    done
done

for db in music video photo; do
    for testfile in `/usr/bin/psql mediaserver $DBADMIN -tA -c "select path from $db;"`; do
        if [ ! -f "$testfile" ]; then
            echo "MISSING FILE: $testfile"
            [ -n "$REMOVE" ] && synoindex -d "$testfile"
        fi
    done
done