MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVE…

…RT in subquery

The bug happens because of a combination of unfortunate circumstances:

1. Arguments args[0] and args[2] of Item_func_concat point recursively
(through Item_direct_view_ref's) to the same Item_func_conv_charset.
Both args[0]->args[0]->ref[0] and args[2]->args[0]->ref[0] refer to
this Item_func_conv_charset.

2. When Item_func_concat::args[0]->val_str() is called,
Item_func_conv_charset::val_str() writes its result to
Item_func_conc_charset::tmp_value.

3. Then, for optimization purposes (to avoid copying),
Item_func_substr::val_str() initializes Item_func_substr::tmp_value
to point to the buffer fragment owned by Item_func_conv_charset::tmp_value
Item_func_substr::tmp_value is returned as a result of
Item_func_concat::args[0]->val_str().

4. Due to optimization to avoid memory reallocs,
Item_func_concat::val_str() remembers the result of args[0]->val_str()
in "res" and further uses "res" to collect the return value.

5. When Item_func_concat::args[2]->val_str() is called,
Item_func_conv_charset::tmp_value gets overwritten (see #1),
which effectively overwrites args[0]'s Item_func_substr::tmp_value (see #3),
which effectively overwrites "res" (see #4).

This patch does the following:

a. Changes Item_func_conv_charset::val_str(String *str) to use
   tmp_value and str the other way around. After this change tmp_value
   is used to store a temporary result, while str is used to return the value.
   The fixes the second problem (without SUBSTR):
     SELECT CONCAT(t2,'-',t2) c2
       FROM (SELECT CONVERT(t USING latin1) t2 FROM t1) sub;
   As Item_func_concat::val_str() supplies two different buffers when calling
   args[0]->val_str() and args[2]->val_str(), in the new reduction the result
   created during args[0]->val_str() does not get overwritten by
   args[2]->val_str().

b. Fixing the same problem in val_str() for similar classes

   Item_func_to_base64
   Item_func_from_base64
   Item_func_weight_string
   Item_func_hex
   Item_func_unhex
   Item_func_quote
   Item_func_compress
   Item_func_uncompress
   Item_func_des_encrypt
   Item_func_des_decrypt
   Item_func_conv_charset
   Item_func_reverse
   Item_func_soundex
   Item_func_aes_encrypt
   Item_func_aes_decrypt
   Item_func_buffer

c. Fixing Item_func::val_str_from_val_str_ascii() the same way.
   Now Item_str_ascii_func::ascii_buff is used for temporary value,
   while the parameter passed to val_str() is used to return the result.
   This fixes the same problem when conversion (from ASCII to e.g. UCS2)
   takes place. See the ctype_ucs.test for example queries that returned
   wrong results before the fix.

d. Some Item_func descendand classes had temporary String buffers
   (tmp_value and tmp_str), but did not really use them.
   Removing these temporary buffers from:

   Item_func_decode_histogram
   Item_func_format
   Item_func_binlog_gtid_pos
   Item_func_spatial_collection:

e. Removing Item_func_buffer::tmp_value, because it's not used any more.

f. Renaming Item_func_[un]compress::buffer to "tmp_value",
   for consistency with other classes.

Note, this patch does not fix the following classes
(although they have a similar problem):

   Item_str_conv
   Item_func_make_set
   Item_char_typecast

They have a complex implementations and simple swapping between "tmp_value"
and "str" won't work. These classes will be fixed separately.

mysql-test/r/ctype_ucs.result

@@ -5552,5 +5552,22 @@ SELECT 'a','aa';
 a	aa
 a	aa
 #
+# MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVERT in subquery
+#
+SET NAMES utf8, character_set_connection=ucs2;
+SET @save_optimizer_switch=@@optimizer_switch;
+SET optimizer_switch=_utf8'derived_merge=on';
+CREATE TABLE t1 (t VARCHAR(10) CHARSET latin1);
+INSERT INTO t1 VALUES('abcdefghi');
+SET NAMES utf8, character_set_connection=ucs2;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT HEX(t) t2 FROM t1) sub;
+c2
+616263646566676869-616263646566676869
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT TO_BASE64(t) t2 FROM t1) sub;
+c2
+YWJjZGVmZ2hp-YWJjZGVmZ2hp
+DROP TABLE t1;
+SET optimizer_switch=@save_optimizer_switch;
+#
 # End of 10.0 tests
 #

mysql-test/r/func_concat.result

@@ -149,3 +149,116 @@ CALL p1();
 ########################################40100.000
 DROP PROCEDURE p1;
 # End of 5.1 tests
+#
+# Start of 10.0 tests
+#
+#
+# MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVERT in subquery
+#
+SET @save_optimizer_switch=@@optimizer_switch;
+SET optimizer_switch='derived_merge=on';
+CREATE TABLE t1 (t VARCHAR(10) CHARSET latin1);
+INSERT INTO t1 VALUES('1234567');
+SELECT CONCAT(SUBSTR(t2, 1, 3), SUBSTR(t2, 5)) c1,
+CONCAT(SUBSTR(t2,1,3),'---',SUBSTR(t2,5)) c2
+FROM (SELECT CONVERT(t USING latin1) t2 FROM t1) sub;
+c1	c2
+123567	123---567
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT CONVERT(t USING latin1) t2 FROM t1) sub;
+c2
+1234567-1234567
+DROP TABLE t1;
+CREATE TABLE t1 (t VARCHAR(10) CHARSET latin1);
+INSERT INTO t1 VALUES('1234567');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT CONVERT(t USING latin1) t2 FROM t1) sub;
+c2
+1234567-1234567
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT REVERSE(t) t2 FROM t1) sub;
+c2
+7654321-7654321
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT SOUNDEX(t) t2 FROM t1) sub;
+c2
+-
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT TO_BASE64(t) t2 FROM t1) sub;
+c2
+MTIzNDU2Nw==-MTIzNDU2Nw==
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT WEIGHT_STRING(t) t2 FROM t1) sub;
+c2
+1234567-1234567
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT HEX(t) t2 FROM t1) sub;
+c2
+31323334353637-31323334353637
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT QUOTE(t) t2 FROM t1) sub;
+c2
+'1234567'-'1234567'
+DROP TABLE t1;
+CREATE TABLE t1 (t VARCHAR(32) CHARSET latin1);
+INSERT INTO t1 VALUES(TO_BASE64('abcdefghi'));
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT FROM_BASE64(t) t2 FROM t1) sub;
+c2
+abcdefghi-abcdefghi
+DROP TABLE t1;
+CREATE TABLE t1 (t VARCHAR(32) CHARSET latin1);
+INSERT INTO t1 VALUES(HEX('abcdefghi'));
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT UNHEX(t) t2 FROM t1) sub;
+c2
+abcdefghi-abcdefghi
+DROP TABLE t1;
+CREATE TABLE t1 (t VARCHAR(30) CHARSET latin1);
+INSERT INTO t1 VALUES('test');
+SELECT LENGTH(CONCAT(t2)) c2 FROM (SELECT AES_ENCRYPT(t,'x') t2 FROM t1) sub;
+c2
+16
+SELECT LENGTH(CONCAT(t2,'-',t2)) c2 FROM (SELECT AES_ENCRYPT(t,'x') t2 FROM t1) sub;
+c2
+33
+SELECT LENGTH(CONCAT(t2,'--',t2)) c2 FROM (SELECT AES_ENCRYPT(t,'x') t2 FROM t1) sub;
+c2
+34
+SELECT LENGTH(CONCAT(t2)) c2 FROM (SELECT AES_DECRYPT(AES_ENCRYPT(t,'x'),'x') t2 FROM t1) sub;
+c2
+4
+SELECT LENGTH(CONCAT(t2,'-',t2)) c2 FROM (SELECT AES_DECRYPT(AES_ENCRYPT(t,'x'),'x') t2 FROM t1) sub;
+c2
+9
+SELECT LENGTH(CONCAT(t2,'--',t2)) c2 FROM (SELECT AES_DECRYPT(AES_ENCRYPT(t,'x'),'x') t2 FROM t1) sub;
+c2
+10
+DROP TABLE t1;
+CREATE TABLE t1 (t VARCHAR(64) CHARSET latin1);
+INSERT INTO t1 VALUES('123456789');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT MD5(t) t2 FROM t1) sub;
+c2
+25f9e794323b453885f5181f1b624d0b-25f9e794323b453885f5181f1b624d0b
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT FORMAT(t,2) t2 FROM t1) sub;
+c2
+123,456,789.00-123,456,789.00
+DROP TABLE t1;
+CREATE TABLE t1 (t VARCHAR(32) CHARSET latin1);
+INSERT INTO t1 VALUES('abcdefghi');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT INSERT(t,3,4,'xxx') t2 FROM t1) sub;
+c2
+abxxxghi-abxxxghi
+DROP TABLE t1;
+CREATE TABLE t1 (t VARCHAR(10) CHARSET latin1);
+INSERT INTO t1 VALUES('abcdefghi');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT LEFT(t,10) t2 FROM t1) sub;
+c2
+abcdefghi-abcdefghi
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT RIGHT(t,10) t2 FROM t1) sub;
+c2
+abcdefghi-abcdefghi
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT SUBSTR(t,1,10) t2 FROM t1) sub;
+c2
+abcdefghi-abcdefghi
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT LTRIM(t) t2 FROM t1) sub;
+c2
+abcdefghi-abcdefghi
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT RTRIM(t) t2 FROM t1) sub;
+c2
+abcdefghi-abcdefghi
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT TRIM(t) t2 FROM t1) sub;
+c2
+abcdefghi-abcdefghi
+DROP TABLE t1;
+SET optimizer_switch=@save_optimizer_switch;

mysql-test/r/func_crypt.result

@@ -106,3 +106,21 @@ OLD_PASSWORD(c1)	PASSWORD(c1)
 77023ffe214c04ff	*82E58A2C08AAFE72C8EB523069CD8ADB33F78F58
 DROP TABLE t1;
 End of 5.0 tests
+#
+# Start of 10.0 tests
+#
+#
+# MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVERT in subquery
+#
+SET @save_optimizer_switch=@@optimizer_switch;
+SET optimizer_switch='derived_merge=on';
+CREATE TABLE t1 (t VARCHAR(32) CHARSET latin1);
+INSERT INTO t1 VALUES('abcdefghi');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT ENCRYPT(t,'aa') t2 FROM t1) sub;
+c2
+aaHHlPHAM4sjs-aaHHlPHAM4sjs
+DROP TABLE t1;
+SET optimizer_switch=@save_optimizer_switch;
+#
+# End of 10.0 tests
+#

mysql-test/r/gis.result

@@ -1641,5 +1641,23 @@ AsText(g)
 NULL
 POINT(1 1)
 #
+# MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVERT in subquery
+#
+SET @save_optimizer_switch=@@optimizer_switch;
+SET optimizer_switch='derived_merge=on';
+CREATE TABLE t1 (x INT, y INT);
+INSERT INTO t1 VALUES(0,0);
+SELECT LENGTH(t2) c2 FROM (SELECT ST_BUFFER(POINT(x,y), 0) t2 FROM t1) sub;
+c2
+25
+SELECT LENGTH(CONCAT(t2,'-',t2)) c2 FROM (SELECT ST_BUFFER(POINT(x,y), 0) t2 FROM t1) sub;
+c2
+51
+SELECT LENGTH(CONCAT(t2,'--',t2)) c2 FROM (SELECT ST_BUFFER(POINT(x,y), 0) t2 FROM t1) sub;
+c2
+52
+DROP TABLE t1;
+SET optimizer_switch=@save_optimizer_switch;
+#
 # End 10.0 tests
 #

mysql-test/t/ctype_ucs.test

@@ -923,6 +923,22 @@ SET NAMES utf8, character_set_connection=ucs2;
 SELECT 'a','aa';
 
 
+--echo #
+--echo # MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVERT in subquery
+--echo #
+
+SET NAMES utf8, character_set_connection=ucs2;
+SET @save_optimizer_switch=@@optimizer_switch;
+SET optimizer_switch=_utf8'derived_merge=on';
+CREATE TABLE t1 (t VARCHAR(10) CHARSET latin1);
+INSERT INTO t1 VALUES('abcdefghi');
+SET NAMES utf8, character_set_connection=ucs2;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT HEX(t) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT TO_BASE64(t) t2 FROM t1) sub;
+DROP TABLE t1;
+SET optimizer_switch=@save_optimizer_switch;
+
+
 --echo #
 --echo # End of 10.0 tests
 --echo #

mysql-test/t/func_concat.test

@@ -145,3 +145,94 @@ CALL p1();
 DROP PROCEDURE p1;
 
 --echo # End of 5.1 tests
+
+
+--echo #
+--echo # Start of 10.0 tests
+--echo #
+
+--echo #
+--echo # MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVERT in subquery
+--echo #
+
+SET @save_optimizer_switch=@@optimizer_switch;
+SET optimizer_switch='derived_merge=on';
+
+CREATE TABLE t1 (t VARCHAR(10) CHARSET latin1);
+INSERT INTO t1 VALUES('1234567');
+SELECT CONCAT(SUBSTR(t2, 1, 3), SUBSTR(t2, 5)) c1,
+       CONCAT(SUBSTR(t2,1,3),'---',SUBSTR(t2,5)) c2
+   FROM (SELECT CONVERT(t USING latin1) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT CONVERT(t USING latin1) t2 FROM t1) sub;
+DROP TABLE t1;
+
+# Other functions affected by MDEV-10306
+
+CREATE TABLE t1 (t VARCHAR(10) CHARSET latin1);
+INSERT INTO t1 VALUES('1234567');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT CONVERT(t USING latin1) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT REVERSE(t) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT SOUNDEX(t) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT TO_BASE64(t) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT WEIGHT_STRING(t) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT HEX(t) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT QUOTE(t) t2 FROM t1) sub;
+DROP TABLE t1;
+
+CREATE TABLE t1 (t VARCHAR(32) CHARSET latin1);
+INSERT INTO t1 VALUES(TO_BASE64('abcdefghi'));
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT FROM_BASE64(t) t2 FROM t1) sub;
+DROP TABLE t1;
+
+CREATE TABLE t1 (t VARCHAR(32) CHARSET latin1);
+INSERT INTO t1 VALUES(HEX('abcdefghi'));
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT UNHEX(t) t2 FROM t1) sub;
+DROP TABLE t1;
+
+CREATE TABLE t1 (t VARCHAR(30) CHARSET latin1);
+INSERT INTO t1 VALUES('test');
+SELECT LENGTH(CONCAT(t2)) c2 FROM (SELECT AES_ENCRYPT(t,'x') t2 FROM t1) sub;
+SELECT LENGTH(CONCAT(t2,'-',t2)) c2 FROM (SELECT AES_ENCRYPT(t,'x') t2 FROM t1) sub;
+SELECT LENGTH(CONCAT(t2,'--',t2)) c2 FROM (SELECT AES_ENCRYPT(t,'x') t2 FROM t1) sub;
+SELECT LENGTH(CONCAT(t2)) c2 FROM (SELECT AES_DECRYPT(AES_ENCRYPT(t,'x'),'x') t2 FROM t1) sub;
+SELECT LENGTH(CONCAT(t2,'-',t2)) c2 FROM (SELECT AES_DECRYPT(AES_ENCRYPT(t,'x'),'x') t2 FROM t1) sub;
+SELECT LENGTH(CONCAT(t2,'--',t2)) c2 FROM (SELECT AES_DECRYPT(AES_ENCRYPT(t,'x'),'x') t2 FROM t1) sub;
+DROP TABLE t1;
+
+
+# Functions not affected by MDEV-10306
+# They only had an unused tmp_value, which was removed.
+
+CREATE TABLE t1 (t VARCHAR(64) CHARSET latin1);
+INSERT INTO t1 VALUES('123456789');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT MD5(t) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT FORMAT(t,2) t2 FROM t1) sub;
+DROP TABLE t1;
+
+# Functions not affected by MDEV-10306
+# They already use tmp_value only for internal purposes and
+# return the result in the String passed to val_str()
+
+CREATE TABLE t1 (t VARCHAR(32) CHARSET latin1);
+INSERT INTO t1 VALUES('abcdefghi');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT INSERT(t,3,4,'xxx') t2 FROM t1) sub;
+DROP TABLE t1;
+
+
+# Functions not affected by MDEV-10306
+# They use this code style:
+#   String *res= args[0]->val_str(str);
+#   tmp_value.set(*res, start, end);
+#   return &tmp_value;
+
+CREATE TABLE t1 (t VARCHAR(10) CHARSET latin1);
+INSERT INTO t1 VALUES('abcdefghi');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT LEFT(t,10) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT RIGHT(t,10) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT SUBSTR(t,1,10) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT LTRIM(t) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT RTRIM(t) t2 FROM t1) sub;
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT TRIM(t) t2 FROM t1) sub;
+DROP TABLE t1;
+
+SET optimizer_switch=@save_optimizer_switch;

mysql-test/t/func_crypt.test

@@ -70,3 +70,27 @@ SELECT OLD_PASSWORD(c1), PASSWORD(c1) FROM t1;
 DROP TABLE t1;
 
 --echo End of 5.0 tests
+
+
+--echo #
+--echo # Start of 10.0 tests
+--echo #
+
+--echo #
+--echo # MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVERT in subquery
+--echo #
+
+SET @save_optimizer_switch=@@optimizer_switch;
+SET optimizer_switch='derived_merge=on';
+# ENCRYPT() is not affected by MDEV-10306
+# It already uses tmp_value only for internal purposes and
+# returns the result in the String passed to val_str()
+CREATE TABLE t1 (t VARCHAR(32) CHARSET latin1);
+INSERT INTO t1 VALUES('abcdefghi');
+SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT ENCRYPT(t,'aa') t2 FROM t1) sub;
+DROP TABLE t1;
+SET optimizer_switch=@save_optimizer_switch;
+
+--echo #
+--echo # End of 10.0 tests
+--echo #

mysql-test/t/gis.test

@@ -1502,6 +1502,21 @@ DROP VIEW v1;
 --echo #
 SELECT AsText(g) FROM (SELECT NULL AS g UNION SELECT Point(1,1)) AS t1;
 
+--echo #
+--echo # MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVERT in subquery
+--echo #
+
+SET @save_optimizer_switch=@@optimizer_switch;
+SET optimizer_switch='derived_merge=on';
+CREATE TABLE t1 (x INT, y INT);
+INSERT INTO t1 VALUES(0,0);
+SELECT LENGTH(t2) c2 FROM (SELECT ST_BUFFER(POINT(x,y), 0) t2 FROM t1) sub;
+SELECT LENGTH(CONCAT(t2,'-',t2)) c2 FROM (SELECT ST_BUFFER(POINT(x,y), 0) t2 FROM t1) sub;
+SELECT LENGTH(CONCAT(t2,'--',t2)) c2 FROM (SELECT ST_BUFFER(POINT(x,y), 0) t2 FROM t1) sub;
+DROP TABLE t1;
+SET optimizer_switch=@save_optimizer_switch;
+
+
 --echo #
 --echo # End 10.0 tests
 --echo #

sql/item.h

@@ -830,25 +830,20 @@ class Item {
             store return value of this method.
 
     NOTE
-      Buffer passed via argument  should only be used if the item itself
-      doesn't have an own String buffer. In case when the item maintains
-      it's own string buffer, it's preferable to return it instead to
-      minimize number of mallocs/memcpys.
-      The caller of this method can modify returned string, but only in case
-      when it was allocated on heap, (is_alloced() is true).  This allows
-      the caller to efficiently use a buffer allocated by a child without
-      having to allocate a buffer of it's own. The buffer, given to
-      val_str() as argument, belongs to the caller and is later used by the
-      caller at it's own choosing.
-      A few implications from the above:
-      - unless you return a string object which only points to your buffer
-        but doesn't manages it you should be ready that it will be
-        modified.
-      - even for not allocated strings (is_alloced() == false) the caller
-        can change charset (see Item_func_{typecast/binary}. XXX: is this
-        a bug?
-      - still you should try to minimize data copying and return internal
-        object whenever possible.
+      The caller can modify the returned String, if it's not marked
+      "const" (with the String::mark_as_const() method). That means that
+      if the item returns its own internal buffer (e.g. tmp_value), it
+      *must* be marked "const" [1]. So normally it's preferrable to
+      return the result value in the String, that was passed as an
+      argument. But, for example, SUBSTR() returns a String that simply
+      points into the buffer of SUBSTR()'s args[0]->val_str(). Such a
+      String is always "const", so it's ok to use tmp_value for that and
+      avoid reallocating/copying of the argument String.
+
+      [1] consider SELECT CONCAT(f, ":", f) FROM (SELECT func() AS f);
+      here the return value of f() is used twice in the top-level
+      select, and if they share the same tmp_value buffer, modifying the
+      first one will implicitly modify the second too.
 
     RETURN
       In case of NULL value return 0 (NULL pointer) and set null_value flag