sql: ANSI aliases for LENGTH()

This patch creates aliases CHARACTER_LENGTH() and CHAR_LENGTH()
for LENGTH(). These functions are added because they are described
in ANSI.

Closes #3929

@TarantoolBot document
Title: SQL functions CHAR_LENGTH() and CHARACTER_LENGTH()

The SQL functions CHAR_LENGTH() and CHARACTER_LENGTH() work the
same as the LENGTH() function. They take exactly one argument. If
an argument of type TEXT or can be cast to a TEXT value using
internal casting rules, these functions return the length of the
TEXT value that represents the argument. They throw an error if
the argument cannot be cast to a TEXT value.

src/box/sql/func.c

@@ -1893,6 +1893,9 @@ sqlRegisterBuiltinFunctions(void)
 			  FIELD_TYPE_STRING),
 		FUNCTION2(length, 1, 0, 0, lengthFunc, SQL_FUNC_LENGTH,
 			  FIELD_TYPE_INTEGER),
+		FUNCTION(char_length, 1, 0, 0, lengthFunc, FIELD_TYPE_INTEGER),
+		FUNCTION(character_length, 1, 0, 0, lengthFunc,
+			 FIELD_TYPE_INTEGER),
 		FUNCTION(position, 2, 0, 1, position_func, FIELD_TYPE_INTEGER),
 		FUNCTION(printf, -1, 0, 0, printfFunc, FIELD_TYPE_STRING),
 		FUNCTION(unicode, 1, 0, 0, unicodeFunc, FIELD_TYPE_STRING),

test/sql-tap/func3.test.lua

 #!/usr/bin/env tarantool
 test = require("sqltester")
-test:plan(25)
+test:plan(35)
 
 --!./tcltestrunner.lua
 -- 2010 August 27
@@ -285,5 +285,26 @@ test:do_test(
     end, test:execsql "EXPLAIN SELECT min(1.0+'2.0',4*11)")
 
 
+--
+-- gh-3929: sql: ANSI aliases for LENGTH().
+--
+suits = {}
+suits[1] = {str = '123456789', len = 9}
+suits[2] = {str = '\x80', len = 1}
+suits[3] = {str = '\x61\x62\x63', len = 3}
+suits[4] = {str = '\x7f\x80\x81', len = 3}
+suits[5] = {str = '\x61\xc0', len = 2}
+suits[6] = {str = '\x61\xc0\x80\x80\x80\x80\x80\x80\x80\x80\x80\x80', len = 12}
+suits[7] = {str = '\xc0\x80\x80\x80\x80\x80\x80\x80\x80\x80\x80', len = 11}
+suits[8] = {str = '\x80\x80\x80\x80\x80\x80\x80\x80\x80\x80', len = 10}
+suits[9] = {str = '\x80\x80\x80\x80\x80\xf0\x80\x80\x80\x80', len = 7}
+suits[10] = {str = '\x80\x80\x80\x80\x80\xf0\x80\x80\x80\xff', len = 7}
+
+for k,v in pairs(suits) do
+    test:do_execsql_test(
+        "func3-6."..k,
+        "SELECT CHAR_LENGTH('"..v.str.."'), CHARACTER_LENGTH('"..v.str.."');",
+        {v.len, v.len})
+end
 
 test:finish_test()