Understanding/reading set option show_* on

Question

We have a bad query plan on a query which I can't see why its been chosen.
Its doing table reformatting which we can solve with a set store_index off.
The indexes have been reorg-ed and the stats are up-to-date.
I tried set option show_lio_costing on and looked at the output but it was rather large to understand.
( I also tried set option show_lio_costing on on a 2 table join and the output was over 400 lines long)
Is there a guide to understanding the output of these options ?
What's the most important lines I should be looking for ?
A very useful feature for the the next version of ASE would be a mechanism to present a simple explanation of the query plan selection.

Answer 1

> 1.Are they have composite indexes?
>  If so, how did you update their statistics?
>   update index statistics or update statistics?
>  For ASE 15.7 you have to use 'update index statistics' command.
I've tried update statistics and update index statistics but as the search col doesn't have any index on it then I wouldn't expect this to help.
> 2.Below set options will help you to understand it.
> 2.1.check missing statistics
> set option show_missing_stats on
The code is
     select * from A, B
    where A.col1 > 1234 and A.col2 = "A"
    and A.PK = B.PK
there are no indexes on A.col1 and A.col2 so I wouldn't expect stats on these columns which is shown by
NO STATS on column DB..A.col1
NO STATS on column DB..A.col1
Since almost any column in a table could be involved in a query would it make sense to create statistics on every column ? I can't think this would be realistic to do with the data volumes - but maybe I'm wrong ?
> 2.2.enable below options too
> set statistics io, plancost on
What should I be looking at ?
I'm not expecting ASE to have stats and get this query right in all cases.
I'm trying to understand the output from the set option show* so at least I can explain why ASE has chosen the plan it has. The output is huge and it not easy to read. I'm looking for something which might explain the output and what the most useful parts to look at.