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-Welcome to Wiki!
-\ No newline at end of file
+### Known issues
+
+* On 32-bit systems 64-bit perceptual hash is limited to a 32-bit integer (i.e. only half of the hash is actually stored and used).
+
+### How to check database contents/hash values easily
+
+```sql
+DROP TABLE IF EXISTS phash_check_setof_type cascade;
+CREATE TABLE phash_check_setof_type (created_at timestamp, url text, phash bit(64), owner_uid int, distance int);
+
+CREATE OR REPLACE FUNCTION phash_check_url(text) RETURNS SETOF phash_check_setof_type AS $$
+   SELECT created_at,
+      url,
+      phash::bit(64),
+      owner_uid,
+      unique_1bits(
+         (SELECT phash FROM ttrss_plugin_img_phash_urls WHERE url LIKE '%' ||
+            SUBSTRING($1 FROM position('//' IN $1))),
+         phash) AS distance
+      FROM ttrss_plugin_img_phash_urls
+      ORDER BY distance limit 10
+$$ LANGUAGE SQL;
+```
+
+Use the function like this: 
+
+```sql
+select * from phash_check_url('http://example.com/something.jpg');
+```
+\ No newline at end of file