When-Will-Index-Lose-Effect.md

When will index column lose effect

Operations on index column

explain select * from target_tag where CONCAT('t', name) = 'etname';

Fuzzy searching at beginning

explain select * from target_tag where name like '%etnam%';

All

explain select * from target_tag where name like 'tetnam%';

key type: range

Using OR Condition

Not using index column in one of OR condition

MySQL think scan all table is faster than using index

• searching result amount is relatively close to the whole table amount
  • using index need extra time to get data from index, which is I/O reading.

Using not equal / is not null / not in / not exists

explain select * from target_tag where name != 'testname';