MDEV-31800 Problem with open ranges on prefix blobs keys

don't construct open ranges from prefix blob keys for < (less than)
just as it's already done for > (greater than)

because prefix KEY_PART doesn't create prefix Field for blobs
(see open_table_from_share() near "Create a new field for the key part"),
so stored_field_cmp_to_item() will compare the original field to the
value not taking the prefix length into account.

mysql-test/main/type_blob.result

@@ -1140,5 +1140,18 @@ c	d
 DROP TABLE t1, t2;
 SET @@sql_mode=@save_sql_mode;
 #
+# MDEV-31800 Problem with open ranges on prefix blobs keys
+#
+create table t1 (d text not null, key a (d(6))) ;
+insert into t1 values ('prefix 2' ), ('prefix 0' );
+select d from t1 where d >= 'prefix 1' and d < 'prefix 3';
+d
+prefix 2
+alter table t1 drop index a;
+select d from t1 where d >= 'prefix 1' and d < 'prefix 3';
+d
+prefix 2
+drop table t1;
+#
 # End of 10.4 test
 #

mysql-test/main/type_blob.test

@@ -756,6 +756,16 @@ select * from t2;
 DROP TABLE t1, t2;
 SET @@sql_mode=@save_sql_mode;
 
+--echo #
+--echo # MDEV-31800 Problem with open ranges on prefix blobs keys
+--echo #
+create table t1 (d text not null, key a (d(6))) ;
+insert into t1 values ('prefix 2' ), ('prefix 0' );
+select d from t1 where d >= 'prefix 1' and d < 'prefix 3';
+alter table t1 drop index a;
+select d from t1 where d >= 'prefix 1' and d < 'prefix 3';
+drop table t1;
+
 --echo #
 --echo # End of 10.4 test
 --echo #

sql/opt_range.cc

@@ -1947,18 +1947,25 @@ public:
     Use this constructor if value->save_in_field() went precisely,
     without any data rounding or truncation.
   */
-  SEL_ARG_LT(const uchar *key, Field *field)
+  SEL_ARG_LT(const uchar *key, const KEY_PART *key_part, Field *field)
    :SEL_ARG_LE(key, field)
-  { max_flag= NEAR_MAX; }
+  {
+    // Don't use open ranges for partial key_segments
+    if (!(key_part->flag & HA_PART_KEY_SEG))
+      max_flag= NEAR_MAX;
+  }
   /*
     Use this constructor if value->save_in_field() returned success,
     but we don't know if rounding or truncation happened
     (as some Field::store() do not report minor data changes).
   */
-  SEL_ARG_LT(THD *thd, const uchar *key, Field *field, Item *value)
+  SEL_ARG_LT(THD *thd, const uchar *key,
+             const KEY_PART *key_part, Field *field, Item *value)
    :SEL_ARG_LE(key, field)
   {
-    if (stored_field_cmp_to_item(thd, field, value) == 0)
+    // Don't use open ranges for partial key_segments
+    if (!(key_part->flag & HA_PART_KEY_SEG) &&
+        stored_field_cmp_to_item(thd, field, value) == 0)
       max_flag= NEAR_MAX;
   }
 };
@@ -9032,7 +9039,7 @@ SEL_ARG *Field::stored_field_make_mm_leaf(RANGE_OPT_PARAM *param,
   case SCALAR_CMP_LE:
     DBUG_RETURN(new (mem_root) SEL_ARG_LE(str, this));
   case SCALAR_CMP_LT:
-    DBUG_RETURN(new (mem_root) SEL_ARG_LT(thd, str, this, value));
+    DBUG_RETURN(new (mem_root) SEL_ARG_LT(thd, str, key_part, this, value));
   case SCALAR_CMP_GT:
     DBUG_RETURN(new (mem_root) SEL_ARG_GT(thd, str, key_part, this, value));
   case SCALAR_CMP_GE:
@@ -9061,7 +9068,7 @@ SEL_ARG *Field::stored_field_make_mm_leaf_exact(RANGE_OPT_PARAM *param,
   case SCALAR_CMP_LE:
     DBUG_RETURN(new (param->mem_root) SEL_ARG_LE(str, this));
   case SCALAR_CMP_LT:
-    DBUG_RETURN(new (param->mem_root) SEL_ARG_LT(str, this));
+    DBUG_RETURN(new (param->mem_root) SEL_ARG_LT(str, key_part, this));
   case SCALAR_CMP_GT:
     DBUG_RETURN(new (param->mem_root) SEL_ARG_GT(str, key_part, this));
   case SCALAR_CMP_GE: