This is my second article about hacking MySQL. If you are interested in this topic, you may want to read my previous Hacking MySQL: SIGNAL support (I)
The problem
If I tell you there is a function called TRUNCATE, what do you think it does? which are its arguments?
For me, the obvious behaviour would be something like:
mysql> SELECT TRUNCATE(123.45); +--------------------+ | TRUNCATE(123.45) | +--------------------+ | 123 | +--------------------+ 1 row in set (0.08 sec)
But the actual behaviour is this:
mysql> SELECT TRUNCATE(123.45);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 1
This is because TRUNCATE requires a second argument. The syntax, according to the manual, is:
TRUNCATE(X,D)
Returns the number X, truncated to D decimal places. If D is 0, the result has no decimal point or fractional part. D can be negative to cause D digits left of the decimal point of the value X to become zero.
Wouldn’t be cool if the default value for D were 0 if we don’t specify otherwise?
The solution
This time we only have to touch the parser (sql/sql_yacc.yy). The code for this is quite simple, although you might find some problems on type conversion.
--- mysql-5.1/sql/sql_yacc.yy 2007-05-08 12:25:51.000000000 +0200
+++ mysql-5.1-truncate/sql/sql_yacc.yy 2007-05-08 15:55:42.000000000 +0200
@@ -6741,6 +6741,13 @@
{ $$= new (YYTHD->mem_root) Item_func_replace($3,$5,$7); }
| TRUNCATE_SYM '(' expr ',' expr ')'
{ $$= new (YYTHD->mem_root) Item_func_round($3,$5,1); }
+ | TRUNCATE_SYM '(' expr ')'
+ {
+ THD *thd= YYTHD;
+ Item *i1= new (thd->mem_root) Item_int((char*) "0",(int32) 0,1);
+
+ $$= new (thd->mem_root) Item_func_round($3,i1,1);
+ }
| WEEK_SYM '(' expr ')'
{
THD *thd= YYTHD;
Get the patch: [28304-truncate_default_value.diff]
Like I said, the hardest part was to go through sql/item*.h trying to guess the expected type for Item_func_round and how to cast that type correctly. I should learn how to better debug the MySQL server.
Conclusion
Unfortunately, the bug (#28304) has been marked as to be fixed later but, at least, this helped me on my way to understand MySQL internals.
