Re: Is this planner choice easily explained? - Mailing list pgsql-bugs

>
>Could we see the pg_stats rows for the columns used in these queries?
>(You have done a vacuum analyze recently, I trust...)

Strange you should ask; it is actually taking some effort to persuade them
it's a good idea. They also don't believe that a LIMIT clause affects
strategy choice, so it's an uphill battle. As a result, my strong suspicion
is that there will be no results in pg_stat - but I have asked for the data.

I am mainly interested in understanding the output from this point of view
-- and to understand the choice that has been made in such a 'boundary
case' (when no analyze has been done).

Am I correct in my interpretation that:

     explain SELECT messageblk FROM messageblks WHERE message_idnr =
100::bigint
     ORDER BY messageblk_idnr ;

     Sort  (cost=5793.33..5793.33 rows=1453 width=40)
       ->  Index Scan using messageblks_msg_idx on messageblks
     (cost=0.00..5716.99 rows=1453 width=40)

means it expects to get 1453 rows based on a search for a specific key
(hence why it has a high cost)? Based on other tests I have done, I have
concluded that it assumes a selectivity of 0.5% for non-unique indexes - is
that right?

Whereas with:

     explain SELECT messageblk FROM messageblks WHERE message_idnr =
100::bigint
     ORDER BY messageblk_idnr
     limit 1;

     Limit  (cost=0.00..777.50 rows=1 width=40)
       ->  Index Scan using messageblks_id_idx on messageblks
     (cost=0.00..1129984.15 rows=1453 width=40)

it looks like the high cost on the last line here is based on the number of
pages/tuples in the file, and that the limit is causing 1/1453th of the
cost to be applied. It looks like it gets the 1453 as a basic default
selectivity again.

The problem I have with this is that it seems that the estimate for the
LIMIT is assuming that the first row returned will be the right one; in
fact it has no guarantee that the specified criteria will be satisfied by
the row. Is this assumption likely to be wrong when we do have stats? ISTM
that a LIMIT with a WHERE clause that is not satisfied by any index used in
the search should not be deemed to reduce the result set size as
drastically. In fact, it may actually have to scan the entire  result set
to get the row. Err...I think 8-).




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