Обсуждение: Stuck using Sequential Scan

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I have a problem where I have the table format listed below.  I have the
primary key tsyslog_id and the index built against it.  However, when I
select a unique row, it will only ever do a seq scan even after I turn off
all other types except indexscan.  I understand you cannot fully turn off seq
scan.

Syslog_TArchive size:  1,426,472,960 bytes
syslog_tarchive_pkey size:  132,833,280 bytes
archhost_idx size:  300,802,048 bytes
tarchdatetime_idx size:  159,293,440 bytes
tarchhostid_idx size:  362,323,968 bytes

I cannot run vacuum more than once a day because of its heavy IO penalty.  I
run analyze once an hour.  However, if I run analyze then explain, I see no
difference in the planners decisions.  What am I missing?


TSyslog=# \d syslog_tarchive;
                                        Table "public.syslog_tarchive"
   Column   |          Type          |
Modifiers
- ------------+------------------------+-------------------------------------------------------------------------
 tsyslog_id | bigint                 | not null default
nextval('public.syslog_tarchive_tsyslog_id_seq'::text)
 facility   | integer                |
 severity   | integer                |
 date       | date                   |
 time       | time without time zone |
 host       | character varying(128) |
 message    | text                   |
Indexes:
    "syslog_tarchive_pkey" primary key, btree (tsyslog_id)
    "archhost_idx" btree (host)
    "tarchdatetime_idx" btree (date, "time")
    "tarchhostid_idx" btree (tsyslog_id, host)

TSyslog=# explain select * from tsyslog where tsyslog_id=431650835;
                               QUERY PLAN
- -------------------------------------------------------------------------
 Seq Scan on tsyslog  (cost=100000000.00..100000058.20 rows=2 width=187)
   Filter: (tsyslog_id = 431650835)
(2 rows)

- --

- --------------------------------------------------
Jeremy M. Guthrie        jeremy.guthrie@berbee.com
Senior Network Engineer        Phone: 608-298-1061
Berbee                           Fax: 608-288-3007
5520 Research Park Drive         NOC: 608-298-1102
Madison, WI 53711
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On Tue, 2004-09-07 at 22:32, Jeremy M. Guthrie wrote:
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>
> I have a problem where I have the table format listed below.  I have the
> primary key tsyslog_id and the index built against it.  However, when I
> select a unique row, it will only ever do a seq scan even after I turn off
> all other types except indexscan.  I understand you cannot fully turn off seq
> scan.
...
> I cannot run vacuum more than once a day because of its heavy IO penalty.  I
> run analyze once an hour.  However, if I run analyze then explain, I see no
> difference in the planners decisions.  What am I missing?
>
>
> TSyslog=# \d syslog_tarchive;
>                                         Table "public.syslog_tarchive"
>    Column   |          Type          |
> Modifiers
> - ------------+------------------------+-------------------------------------------------------------------------
>  tsyslog_id | bigint                 | not null default
...
>
> TSyslog=# explain select * from tsyslog where tsyslog_id=431650835;

That constant is INTEGER, whereas the column is BIGINT; there is no
automatic conversion in this case, so the planner does not realise the
index is usable for this query (I think 8.0 solves this).

Try: select * from tsyslog where tsyslog_id=431650835::BIGINT;

--
Oliver Elphick                                          olly@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
GPG: 1024D/A54310EA  92C8 39E7 280E 3631 3F0E  1EC0 5664 7A2F A543 10EA
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