Strange use of the index in Mysql - sql

Question

Explain
SELECT `feed_objects`.*
FROM `feed_objects`
WHERE (`feed_objects`.feed_id IN
( 165,160,159,158,157,153,152,151,150,149,148,147,129,128,127,126,125,124,122,
121, 120,119,118,117,116,115,114,113,111,110)) ;
+----+-------------+--------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | feed_objects | ALL | by_feed_id | NULL | NULL | NULL | 188 | Using where |
+----+-------------+--------------+------+---------------+------+---------+------+------+-------------+
Not used index by_feed_id
But when I point less than the values in the WHERE - everything is working right
Explain
SELECT `feed_objects`.*
FROM `feed_objects`
WHERE (`feed_objects`.feed_id IN
(165,160,159,158,157,153,152,151,150,149,148,147,129,128,127,125,124)) ;
+----+-------------+--------------+-------+---------------+------------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+------------+---------+------+------+-------------+
| 1 | SIMPLE | feed_objects | range | by_feed_id | by_feed_id | 9 | NULL | 18 | Using where |
+----+-------------+--------------+-------+---------------+------------+---------+------+------+-------------+
Used index by_feed_id
What is the problem?

Answer 1

The MySQL optimizer makes a lot of decisions that sometimes look strange. In this particular case, I believe that you have a very small table (188 rows total from the looks of the first EXPLAIN), and that is affecting the optimizer's decision.
The "How MySQL Uses Indexes" manual pages offers this snippet of info:
Sometimes MySQL does not use an index,
even if one is available. One
circumstance under which this occurs
is when the optimizer estimates that
using the index would require MySQL to
access a very large percentage of the
rows in the table. (In this case, a
table scan is likely to be much faster
because it requires fewer seeks.)
Because the number of ids in your first WHERE clause is relatively large compared to the table size, MySQL has determined that it is faster to scan the data than to consult the index, as the data scan is likely to result in less disk access time.
You could test this by adding rows to the table and re-running the first EXPLAIN query. At a certain point, MySQL will start using the index for it as well as the second query.