What can we learn from EXPLAIN SELECT?

By looking at this


1 - Can we tell that the query is well optimized as the select types are all "simple"? If not what the main purpose of information in this column?

2 - What do we learn from "rows" column? Is 1795 too many rows? If so how do we know how we can minimize it?

3 - What is the benefits of information in "possible_keys" and "Extra" columns?

It would help if you also posted your tables structure and query.

One thing that is not good is "using filesort".
The field 'Possible Index' can give you an hint on which of the fields it could benefit to have an index added.

nope, but good guess

"possible_keys" lists all the indexes that it looked at that might help optimize this query, and "key" is the one it chose

By looking at explain can you tell why this query takes a long time to load?

Will it be possible of you to post following information:
- table(s) structure
- query
- the actual time it takes
- number of total rows


@r937
Ah well, but thanks for the explanation. Always nice to learn something new.

I appreciate the posts Fujin and Rudy but your posts are not answering any of my questions

1. not necessarily

yes, you want SIMPLE, but i've never seen anything else

2. 1795 is not "too many" if that's how many meet the query criteria!!

3. possible_keys tells you whether there are any indexes that help improve performance beyond the slow-as-cold-glue table scan (the index it chooses is in the key column)

extra is the most important column, it will tell you when a sort is being done (sorts are very expensive)


by the way, analyzing an EXPLAIN out of context, with no idea of what the query is asking for or what tables look like, is largely guesswork