Greetings,
I did the following two explain select ... According to whether I put
the single quotation mark or not, I will get totally different results:
1. Without single quotation mark:
explain SELECT * FROM test T1 force index (idx_test)
WHERE *T1.STUDID = 0999999 AND T1.Prog_link = 6666*;
+-------+--------+------------------------+---------+---------+---------------------------------+-------+-------------+
| table | type | possible_keys | key | key_len |
ref | rows | Extra |
+-------+--------+------------------------+---------+---------+---------------------------------+-------+-------------+
| T1 | ALL | idx_test | NULL | NULL |
NULL | 91230 | Using where |
+-------+--------+------------------------+---------+---------+---------------------------------+-------+-------------+
2. With single quotation mark:
explain SELECT * FROM test T1 force index (idx_test)
WHERE *T1.STUDID = '0999999**' AND T1.Prog_link = '6666'*;
+-------+--------+------------------------+----------+---------+---------------------------------+------+-------------+
| table | type | possible_keys | key | key_len |
ref | rows | Extra |
+-------+--------+------------------------+----------+---------+---------------------------------+------+-------------+
| T1 | ref | idx_test | idx_test | 7 |
const | 27 | Using where |
+-------+--------+------------------------+----------+---------+---------------------------------+------+-------------+
Note: . create index idx_test on test(studid, prog_link);
. in test table, studid is varchar(7), prog_link is varchar(4).
Thanks a lot!
Emi
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