sql performance and cache

Hello all

I have two very similar queries which I need to execute. They both have
exactly the same from / where conditions. When I execute the first, it takes
about 16 seconds. The second is executed almost immediately after, it takes
13 seconds. In short, I'd like to know why the query result isn't being
cached and any ideas on how to improve the execution.

The first query attempts to find the maximum size of an array in the result
set- the field is called "level". IT contains anything between 1 and 10
integers. I just need to know what the largest size is. I do this to find
out the maximum size of the "level" array.

"max(replace(split_part(array_dims(level),':',2),']','')::int)"

I know this is big and ugly but is there any better way of doing it ?

The second query just returns the result set - it has exactly the same
FROM/Where clause.

OK - so I could execute the query once, and get the maximum size of the
array and the result set in one. I know what I am doing is less than optimal
but I had expected the query results to be cached. So the second execution
would be very quick. So why aren't they ? I have increased my cache size -
shared_buffers is 2000 and I have doubled the default max_fsm... settings
(although I am not sure what they do). sort_mem is 8192.

The from / where is

FROM oscar_node N, oscar_point P
where N."GEOM_ID_OF_POINT" = P."POINT_ID"
and N."TILE_REF" = P."TILE_REF"
and N."TILE_REF" in ('TQ27NE','TQ28SE','TQ37NW','TQ38SW')
and P."TILE_REF" in ('TQ27NE','TQ28SE','TQ37NW','TQ38SW')
and P."FEAT_CODE" = 3500
and P.wkb_geometry && GeometryFromText('BOX3D(529540.0 179658.88,530540.0
180307.12)'::box3d,-1)

oscar_node and oscar_point both have about 3m rows. PK on oscar_node is
composite of "TILE_REF" and "NODE_ID". PK on oscar_point is "TILE_REF" and
"POINT_ID". The tables are indexed on feat_code and I have an index on
wkb_geometry. (This is a GIST index). I have increased the statistics size
and done the analyze command.

Here is my explain plan

 Nested Loop  (cost=0.00..147.11 rows=1 width=148)
   Join Filter: ("inner"."GEOM_ID_OF_POINT" = "outer"."POINT_ID")
   ->  Index Scan using gidx_oscar_point on oscar_point p  (cost=0.00..61.34
rows=1 width=57)
         Index Cond: (wkb_geometry && 'SRID=-1;BOX3D(529540 179658.88
0,530540 180307.12 0)'::geometry)
         Filter: ((("TILE_REF" = 'TQ27NE'::bpchar) OR ("TILE_REF" =
'TQ28SE'::bpchar) OR ("TILE_REF" = 'TQ37NW'::bpchar) OR ("TILE_REF" =
'TQ38SW'::bpchar)) AND ("FEAT_CODE" = 3500))
   ->  Index Scan using idx_on_tile_ref on oscar_node n  (cost=0.00..85.74
rows=2 width=91)
         Index Cond: (n."TILE_REF" = "outer"."TILE_REF")
         Filter: (("TILE_REF" = 'TQ27NE'::bpchar) OR ("TILE_REF" =
'TQ28SE'::bpchar) OR ("TILE_REF" = 'TQ37NW'::bpchar) OR ("TILE_REF" =
'TQ38SW'::bpchar))


I am seeing this message in my logs.

"bt_fixroot: not valid old root page"

Maybe this is relevant to my performance problems.

I know this has been a long message but I would really appreciate any
performance tips.

Thanks


Chris