Fixed problem with auto_increment key as not first key part

Docs/manual.texi:
  Changelog

Docs/manual.texi

@@ -46928,6 +46928,9 @@ not yet 100% confident in this code.
 @appendixsubsec Changes in release 3.23.52
 @itemize @bullet
 @item
+Fixed bug with creating an auto-increment value on second part of a
+@code{UNIQUE()} key where first part could contain NULL values.
+@item
 Don't write slave-timeout reconnects to the error log.
 @item
 Fixed bug with slave net read timeouting

sql/key.cc

@@ -59,7 +59,7 @@ int find_ref_key(TABLE *table,Field *field, uint *key_length)
     {
       if (key_part->offset == fieldpos)
 	return(i);			/* Use this key */
-      *key_length+=key_part->length;
+      *key_length+=key_part->store_length;
     }
   }
   return(-1);					/* No key is ok */

sql/table.cc

@@ -346,17 +346,7 @@ int openfrm(const char *name, const char *alias, uint db_stat, uint prgflag,
       }
     }
     if (reg_field->unireg_check == Field::NEXT_NUMBER)
-    {
-      if ((int) (outparam->next_number_index= (uint)
-		 find_ref_key(outparam,reg_field,
-			      &outparam->next_number_key_offset)) < 0)
-	reg_field->unireg_check=Field::NONE;	/* purecov: inspected */
-      else
-      {
-	outparam->found_next_number_field=reg_field;
-	reg_field->flags|=AUTO_INCREMENT_FLAG;
-      }
-    }
+      outparam->found_next_number_field= reg_field;
     if (outparam->timestamp_field == reg_field)
       outparam->timestamp_field_offset=i;
     if (use_hash)
@@ -507,6 +497,19 @@ int openfrm(const char *name, const char *alias, uint db_stat, uint prgflag,
       bfill(outparam->null_flags+outparam->rec_buff_length*2,null_length,255);
   }
 
+  if ((reg_field=outparam->found_next_number_field))
+  {
+    if ((int) (outparam->next_number_index= (uint)
+	       find_ref_key(outparam,reg_field,
+			    &outparam->next_number_key_offset)) < 0)
+    {
+      reg_field->unireg_check=Field::NONE;	/* purecov: inspected */
+      outparam->found_next_number_field=0;
+    }
+    else
+      reg_field->flags|=AUTO_INCREMENT_FLAG;
+  }
+
   if (outparam->blob_fields)
   {
     Field **ptr;