Bug #45262: Bad effects with CREATE TABLE and DECIMAL

Using DECIMAL constants with more than 65 digits in CREATE 
TABLE ... SELECT led to bogus errors in release builds or 
assertion failures in debug builds. 
 
The problem was in inconsistency in how DECIMAL constants and 
fields are handled internally. We allow arbitrarily long 
DECIMAL constants, whereas DECIMAL(M,D) columns are limited to 
M<=65 and D<=30. my_decimal_precision_to_length() was used in 
both Item and Field code and truncated precision to 
DECIMAL_MAX_PRECISION when calculating value length without 
adjusting precision and decimals. As a result, a DECIMAL 
constant with more than 65 digits ended up having length less 
than precision or decimals which led to assertion failures. 
 
Fixed by modifying my_decimal_precision_to_length() so that 
precision is truncated to DECIMAL_MAX_PRECISION only for Field 
object which is indicated by the new 'truncate' parameter. 
 
Another inconsistency fixed by this patch is how DECIMAL 
constants and expressions are handled for CREATE ... SELECT. 
create_tmp_field_from_item() (which is used for constants) was 
changed as a part of the bugfix for bug #24907 to handle long 
DECIMAL constants gracefully. Item_func::tmp_table_field() 
(which is used for expressions) on the other hand was still 
using a simplistic approach when creating a Field_new_decimal 
from a DECIMAL expression.

sql/sql_select.cc

@@ -9087,13 +9087,17 @@ static Field *create_tmp_field_from_item(THD *thd, Item *item, TABLE *table,
         +1: for decimal point
       */
 
-      overflow= my_decimal_precision_to_length(intg + dec, dec,
-                                               item->unsigned_flag) - len;
+      const int required_length=
+        my_decimal_precision_to_length(intg + dec, dec,
+                                                     item->unsigned_flag);
+
+      overflow= required_length - len;
 
       if (overflow > 0)
         dec= max(0, dec - overflow);            // too long, discard fract
       else
-        len -= item->decimals - dec;            // corrected value fits
+        /* Corrected value fits. */
+        len= required_length;
     }
 
     new_field= new Field_new_decimal(len, maybe_null, item->name,