MySQL Indexes and Keys
Syntax
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-- Create simple index
CREATE INDEX index_name ON table_name(column_name1 [, column_name2, ...])
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-- Create unique index
CREATE UNIQUE INDEX index_name ON table_name(column_name1 [, column_name2, ...]
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-- Drop index
DROP INDEX index_name ON tbl_name [algorithm_option | lock_option] ...
algorithm_option: ALGORITHM [=] {DEFAULT|INPLACE|COPY}
lock_option: LOCK [=] {DEFAULT|NONE|SHARED|EXCLUSIVE}
Remarks
Concepts
An index in a MySQL table works like an index in a book.
Let's say you have a book about databases and you want to find some information about, say, storage. Without an index (assuming no other aid, such as a table of contents) you'd have to go through the pages one by one, until you found the topic (that's a "full table scan"). On the other hand, an index has a list of keywords, so you'd consult the index and see that storage is mentioned on pages 113-120, 231, and 354. Then you could flip to those pages directly, without searching (that's a search with an index, somewhat faster).
Of course, the usefulness of the index depends on many things - a few examples, using the simile above:
- If you had a book on databases and indexed the word "database", you might see that it's mentioned on pages 1-59, 61-290, and 292-400. That's a lot of pages, and in such a case, the index is not much help and it might be faster to go through the pages one by one. (In a database, this is "poor selectivity".)
- For a 10-page book, it makes no sense to make an index, as you may end up with a 10-page book prefixed by a 5-page index, which is just silly - just scan the 10 pages and be done with it.
- The index also needs to be useful - there's generally no point to indexing, for example, the frequency of the letter "L" per page.
