fix(tianmu): default value of the field take unaffect in load #1865

Cause:
  in the function ParsingStrategy::ParseResult ParsingStrategy::GetOneRow
field->val_str(str) cannot distinguish 0 and NULL value.
Solution:
  Check whether field's default value is NULL.

mysql-test/suite/tianmu/r/issue1865.result

@@ -0,0 +1,28 @@
+DROP DATABASE IF EXISTS issue1865_test_db;
+CREATE DATABASE issue1865_test_db;
+create table t1 (a int default 100, b int, c varchar(60))engine=tianmu;
+load data infile 'MYSQL_TEST_DIR/suite/tianmu/std_data/issue1865.dat' into table t1 (a, c);
+select * from t1;
+a	b	c
+NULL	NULL	10
+NULL	NULL	15
+alter table t1 alter column b drop default;
+alter table t1 alter column b set default 10;
+load data infile 'MYSQL_TEST_DIR/suite/tianmu/std_data/issue1865.dat' into table t1 (a, c);
+select * from t1;
+a	b	c
+NULL	NULL	10
+NULL	NULL	15
+NULL	10	10
+NULL	10	15
+alter table t1 modify c text;
+load data infile 'MYSQL_TEST_DIR/suite/tianmu/std_data/issue1865.dat' into table t1 (a, c);
+select * from t1;
+a	b	c
+NULL	NULL	10
+NULL	NULL	15
+NULL	10	10
+NULL	10	15
+NULL	10	10
+NULL	10	15
+DROP DATABASE issue1865_test_db;

mysql-test/suite/tianmu/std_data/issue1865.dat

@@ -0,0 +1,2 @@
+\N	10
+\N	15

mysql-test/suite/tianmu/t/issue1865.test

@@ -0,0 +1,27 @@
+--source include/have_tianmu.inc
+
+--disable_warnings
+DROP DATABASE IF EXISTS issue1865_test_db;
+--enable_warnings
+
+CREATE DATABASE issue1865_test_db;
+
+create table t1 (a int default 100, b int, c varchar(60))engine=tianmu;
+
+--replace_result $MYSQL_TEST_DIR MYSQL_TEST_DIR
+eval load data infile '$MYSQL_TEST_DIR/suite/tianmu/std_data/issue1865.dat' into table t1 (a, c);
+select * from t1;
+
+alter table t1 alter column b drop default;
+alter table t1 alter column b set default 10;
+
+--replace_result $MYSQL_TEST_DIR MYSQL_TEST_DIR
+eval load data infile '$MYSQL_TEST_DIR/suite/tianmu/std_data/issue1865.dat' into table t1 (a, c);
+select * from t1;
+
+alter table t1 modify c text;
+--replace_result $MYSQL_TEST_DIR MYSQL_TEST_DIR
+eval load data infile '$MYSQL_TEST_DIR/suite/tianmu/std_data/issue1865.dat' into table t1 (a, c);
+select * from t1;
+
+DROP DATABASE issue1865_test_db;

storage/tianmu/loader/parsing_strategy.cpp

@@ -433,9 +433,12 @@ ParsingStrategy::ParseResult ParsingStrategy::GetOneRow(const char *const buf, s
     if (!first_row_prepared_) {
       std::string field_name(field->field_name);
 
-      str = new (thd_->mem_root) String(MAX_FIELD_WIDTH);
-      String *res = field->val_str(str);
-      DEBUG_ASSERT(res);
+      str = new (thd_->mem_root) String();
+      if (!field->is_null()) {
+        str->real_alloc(MAX_FIELD_WIDTH);
+        String *res = field->val_str(str);
+        DEBUG_ASSERT(res);
+      }
       vec_field_Str_list_.push_back(str);
       vec_field_num_to_index_.push_back(0);
       map_field_name_to_index_[field_name] = i;