Message-ID: <430F2AF3.1080704@rentec.com>
Date: Fri, 26 Aug 2005 10:45:07 -0400
From: Alan Stange <stange@rentec.com>
Reply-To: stange@rentec.com
Organization: Renaissance Technologies Corp.
User-Agent: Mozilla Thunderbird 1.0+ (X11/20050712)
MIME-Version: 1.0
To: pgsql-performance@postgresql.org
Subject: difference in plan between 8.0 and 8.1?
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Hello all,

I was hoping someone could explain the plan for a statement.  

We have a table with a column of longs being used as an index.  The 
query plan in 8.0 was like this:

# explain select distinct timeseriesid from tbltimeseries where 
timeseriesid > 0 order by timeseriesid;
SET
                                              QUERY PLAN
-------------------------------------------------------------------------------------------------------
 Unique  (cost=0.00..15065908.60 rows=10854026 width=8)
   ->  Index Scan using idx_timeseris on tbltimeseries  
(cost=0.00..15038773.53 rows=10854026 width=8)
         Index Cond: (timeseriesid > 0)
(3 rows)


In 8.1, (using the same database after a dump+restore+vacuum+analyze) I 
get the following:
# explain select distinct timeseriesid from tbltimeseries where 
timeseriesid > 0 order by timeseriesid;
                                            QUERY PLAN
---------------------------------------------------------------------------------------------------
 Unique  (cost=2717137.08..2771407.21 rows=10854026 width=8)
   ->  Sort  (cost=2717137.08..2744272.14 rows=10854026 width=8)
         Sort Key: timeseriesid
         ->  Bitmap Heap Scan on tbltimeseries  
(cost=48714.09..1331000.42 rows=10854026 width=8)
               Recheck Cond: (timeseriesid > 0)
               ->  Bitmap Index Scan on idx_timeseris  
(cost=0.00..48714.09 rows=10854026 width=0)
                     Index Cond: (timeseriesid > 0)
(7 rows)


I'm hoping someone can explain the new query plan (as I'm not sure I 
understand what it is doing).

Thanks!

-- Alan