MDEV-18922 Alter on long unique varchar column makes result null

Don't add long key into share->keys_for_keyread

mysql-test/main/long_unique_bugs.result

@@ -129,3 +129,17 @@ Comment
 Index_comment	
 ALTER TABLE t1 ADD INDEX (pk);
 DROP TABLE t1;
+CREATE TABLE t1 (b int, a  varchar(4000));
+INSERT INTO t1 VALUES (1, 2),(2,3),(3,4);
+ALTER TABLE t1 ADD UNIQUE INDEX (a);
+SELECT * FROM t1;
+b	a
+1	2
+2	3
+3	4
+SELECT a FROM t1;
+a
+2
+3
+4
+drop table t1;

mysql-test/main/long_unique_bugs.test

@@ -150,3 +150,13 @@ CREATE TABLE t1 (pk INT, PRIMARY KEY USING HASH (pk)) ENGINE=InnoDB;
 --query_vertical show keys from t1;
 ALTER TABLE t1 ADD INDEX (pk);
 DROP TABLE t1;
+
+#
+# MDEV-18922 Alter on long unique varchar column makes result null
+#
+CREATE TABLE t1 (b int, a  varchar(4000));
+INSERT INTO t1 VALUES (1, 2),(2,3),(3,4);
+ALTER TABLE t1 ADD UNIQUE INDEX (a);
+SELECT * FROM t1;
+SELECT a FROM t1;
+drop table t1;

sql/table.cc

@@ -2684,7 +2684,8 @@ int TABLE_SHARE::init_from_binary_frm_image(THD *thd, bool write,
         if (i == 0)
           field->key_start.set_bit(key);
         if (field->key_length() == key_part->length &&
-            !(field->flags & BLOB_FLAG))
+            !(field->flags & BLOB_FLAG) &&
+            key_info->algorithm != HA_KEY_ALG_LONG_HASH)
         {
           if (handler_file->index_flags(key, i, 0) & HA_KEYREAD_ONLY)
           {