MySQL may have noticed it was impossible with the current data he had
using the index on that field, but in the future it may become valid, so
I wouldn't necessary eliminate it if it's deemed crucial. A query that
returns no results is equally as useful as one that returns many.
I'm not arguing about the index, I'm saying you can't use that
particular example to try and optimize the db.
In mysql you cannot use explain on a query that returns no results.
Every time it will give the "impossible where noticed" error.
mysql> create table test (id int);
Query OK, 0 rows affected (0.06 sec)
mysql> insert into test(id) values (1);
Query OK, 1 row affected (0.06 sec)
mysql> explain select * from test where id=1\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test
type: system
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 1
Extra:
1 row in set (0.00 sec)
mysql> explain select * from test where id=2\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: Impossible WHERE noticed after reading const tables
1 row in set (0.01 sec)
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