Olaf Faaland wrote:
Hi,
I am currently using mysql 4.0.18 as distributed with red hat Linux. I find
when I perform a select on a varchar(30) field, the index is used only if I
have quoted the value in the where clause. Otherwise, mysql performs a
table scan.
A varchar(30) field is a string, so only comparisons to strings really make
sense. If you don't quote the comparison value in the WHERE clause, you're
asking for trouble. First, the unquoted value must make sense. There are 3
possibilities: it's a number, it's a function, or it's a column name.
Hence, you can get away with
WHERE varchar_field = 170807
but probably not
WHERE varachar_field = dogs
(unless you have a column named "dogs").
I expect you know all that. My point is, why aren't you quoting the
comparison string?
The queries in question are:
This query uses the index:
mysql> explain
-> select itran_log_date, itran_log_actionid from itran_log where
-> itran_log_actionid = "170807";
This is a string column compared to a string constant, so the index can be used.
This query performs a table scan:
mysql> explain
-> select itran_log_date, itran_log_actionid from itran_log where
-> itran_log_actionid = 170807;
This is a string compared to a number. You are asking mysql to make an
implicit conversion so it can make the comparison. You expected the number
to be converted to a string, but that's not how it works -- it's the other
way around, the string is converted to a number. Why? Because many strings
convert to the same number. Consider:
mysql> SELECT '17' + 0, '17.0' + 0, '+17' + 0, '17,345' + 0, '17 dogs' + 0;
+----------+------------+-----------+--------------+---------------+
| '17' + 0 | '17.0' + 0 | '+17' + 0 | '17,345' + 0 | '17 dogs' + 0 |
+----------+------------+-----------+--------------+---------------+
| 17 | 17 | 17 | 17 | 17 |
+----------+------------+-----------+--------------+---------------+
1 row in set (0.00 sec)
The map from string to number is many-to-one, so the only safe course is to
convert the string to a number, then do a numeric comparison. Of course,
that renders the string index useless, so the full table scan is required.
My question is this: is the issue here that mysql is converting every single
itran_log_actionid value, from all 1.5 million rows, and hence the index is
not useful and not used?
Yes.
My initial assumption was that the constant value
170807 in the second query, would be converted to text before the query was
executed, and so the index could be used. This does not seem to be the
case.
Right, it's not.
I ask both for my own edification, and also because it seems to me this
should be mentioned in the manual for newbies like myself.
I thought it was mentioned in the manual, though I must admit I didn't find
it just now after a brief search.
I'm wondering, however, if we're really approaching this from the right
angle. Your column is named itran_log_actionid, and it seems to contain
numbers such as 170807. The obvious question, then, is why is
itran_log_actionid a VARCHAR(30) instead of one of the INT types? If it
were, you wouldn't need to quote the constant in the WHERE clause, and the
comparison would be numeric (faster than string) with no type conversion
thanks,
Olaf
Michael
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