Re: Help with query plan inconsistencies

I'm going to ask because someone else surely will:

Do you regularily vacuum/analyze the database?

Woody Woodring wrote:
> Hello,
>
> I am using postgres 7.4.2 as a backend for geocode data for a mapping
> application. My question is why can't I get a consistent use of my indexes
> during a query, I tend to get a lot of seq scan results.
>
> I use a standard query:
>
> SELECT lat, long, mac, status FROM (
> SELECT text(mac) as mac, lat long, CASE status WHEN 0 THEN 0 WHEN 1 THEN
> 1 ELSE -1 END
> as status FROM cable_billing LEFT OUTER JOIN davic USING(mac) WHERE
> boxtype='d'
> )AS FOO WHERE (long>=X1) AND (long<=X2) AND (lat>=Y1) AND (lat<=Y2)
>
> Where X1,X2,Y1,Y2 are the coordinates for the rectangle of the map viewing
> area.
>
> QUERY PLAN #1 & #2 are from when I get a view from 10 miles out, sometimes
> it uses the index(#1) and most of the time not(#2). I do run into plans
> that seq scan both sides of the join.
>
> QUERY PLAN #3 is when I view from 5 miles out, and I have much greater
> chance of getting index scans ( about 90% of the time).
>
> I have listed information about the database below.
>
> Cable_billing ~500,000 rows updated once per day
> Davic ~500,000 rows, about 100 rows update per minute
>
> Any info or suggestions would be appreciated.
>
> Woody
>
>
> twc-ral-overview=# \d cable_billing;
> Table "public.cable_billing"
> Column | Type | Modifiers
> -----------------+------------------------+-----------
> cable_billingid | integer | not null
> mac | macaddr | not null
> account | integer |
> number | character varying(10) |
> address | character varying(200) |
> region | character varying(30) |
> division | integer |
> franchise | integer |
> node | character varying(10) |
> lat | numeric |
> long | numeric |
> trunk | character varying(5) |
> ps | character varying(5) |
> fd | character varying(5) |
> le | character varying(5) |
> update | integer |
> boxtype | character(1) |
> Indexes: cable_billing_pkey primary key btree (mac),
> cable_billing_account_index btree (account),
> cable_billing_lat_long_idx btree (lat, long),
> cable_billing_node_index btree (node),
> cable_billing_region_index btree (region)
>
> twc-ral-overview=# \d davic
> Table "public.davic"
> Column | Type | Modifiers
> ---------+-----------------------+-----------
> davicid | integer | not null
> mac | macaddr | not null
> source | character varying(20) |
> status | smallint |
> updtime | integer |
> type | character varying(10) |
> avail1 | integer |
> Indexes: davic_pkey primary key btree (mac)
>
>
>
> twc-ral-overview=# vacuum analyze;
> VACUUM
> twc-ral-overview=# explain analyze SELECT lat, long, mac, status FROM
> (SELECT text(mac) as mac, lat, long, CASE status WHEN 0 THEN 0 WHEN 1 THEN 1
> ELSE -1 END as status FROM cable_billing LEFT OUTER JOIN davic USING(mac)
> WHERE boxtype='d') AS foo WHERE (long>=-78.70723462816063) AND
> (long<=-78.53096764204116) AND (lat>=35.57411187866667) AND
> (lat<=35.66366331376857);
> QUERY PLAN #1
>
> ----------------------------------------------------------------------------
> ----------------------------------------------------------------------------
> -----
> Nested Loop Left Join (cost=0.00..23433.18 rows=1871 width=34) (actual
> time=0.555..5095.434 rows=3224 loops=1)
> -> Index Scan using cable_billing_lat_long_idx on cable_billing
> (cost=0.00..12145.85 rows=1871 width=32) (actual time=0.431..249.931
> rows=3224 loops=1)
> Index Cond: ((lat >= 35.57411187866667) AND (lat <=
> 35.66366331376857) AND (long >= -78.70723462816063) AND (long <=
> -78.53096764204116))
> Filter: (boxtype = 'd'::bpchar)
> -> Index Scan using davic_pkey on davic (cost=0.00..6.01 rows=1
> width=8) (actual time=1.476..1.480 rows=1 loops=3224)
> Index Cond: ("outer".mac = davic.mac)
> Total runtime: 5100.028 ms
> (7 rows)
>
>
>
> twc-ral-overview=# vacuum analyze;
> VACUUM
> twc-ral-overview=# explain analyze SELECT lat, long, mac, status FROM
> (SELECT text(mac) as mac, lat, long, CASE status WHEN 0 THEN 0 WHEN 1 THEN 1
> ELSE -1 END as status FROM cable_billing LEFT OUTER JOIN davic USING(mac)
> WHERE boxtype='d') AS foo WHERE (long>=-78.87878592206046) AND
> (long<=-78.70220280717479) AND (lat>=35.71703190638861) AND
> (lat<=35.80658335998006);
> QUERY PLAN #2
>
> ----------------------------------------------------------------------------
> ----------------------------------------------------------------------------
> -------------------
> Nested Loop Left Join (cost=0.00..76468.90 rows=9223 width=34) (actual
> time=0.559..17387.427 rows=19997 loops=1)
> -> Seq Scan on cable_billing (cost=0.00..20837.76 rows=9223 width=32)
> (actual time=0.290..7117.799 rows=19997 loops=1)
> Filter: ((boxtype = 'd'::bpchar) AND (long >= -78.87878592206046)
> AND (long <= -78.70220280717479) AND (lat >= 35.71703190638861) AND (lat <=
> 35.80658335998006))
> -> Index Scan using davic_pkey on davic (cost=0.00..6.01 rows=1
> width=8) (actual time=0.455..0.461 rows=1 loops=19997)
> Index Cond: ("outer".mac = davic.mac)
> Total runtime: 17416.501 ms
> (6 rows)
>
>
>
> twc-ral-overview=# explain analyze SELECT lat, long, mac, status FROM
> (SELECT text(mac) as mac, lat, long, CASE status WHEN 0 THEN 0 WHEN 1 THEN 1
> ELSE -1 END as status FROM cable_billing LEFT OUTER JOIN davic USING(mac)
> WHERE boxtype='d') AS foo WHERE (long>=-78.83419423836857) AND
> (long<=-78.7467945148866) AND (lat>=35.73964586635293) AND
> (lat<=35.783969313080604);
> QUERY PLAN #3
>
> ----------------------------------------------------------------------------
> ----------------------------------------------------------------------------
> -----
> Nested Loop Left Join (cost=0.00..29160.02 rows=2327 width=34) (actual
> time=0.279..510.773 rows=5935 loops=1)
> -> Index Scan using cable_billing_lat_long_idx on cable_billing
> (cost=0.00..15130.08 rows=2326 width=32) (actual time=0.197..274.115
> rows=5935 loops=1)
> Index Cond: ((lat >= 35.73964586635293) AND (lat <=
> 35.783969313080604) AND (long >= -78.83419423836857) AND (long <=
> -78.7467945148866))
> Filter: (boxtype = 'd'::bpchar)
> -> Index Scan using davic_pkey on davic (cost=0.00..6.01 rows=1
> width=8) (actual time=0.021..0.024 rows=1 loops=5935)
> Index Cond: ("outer".mac = davic.mac)
> Total runtime: 516.782 ms
> (7 rows)
>
> ----------------------------------------------------------------------------
> -----------------------------------
>
> iglass Networks
> 211-A S. Salem St.
> (919) 387-3550 x813
> P.O. Box 651
> (919) 387-3570 fax
> Apex, NC 27502
> http://www.iglass.net
>
>
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