I'm trying to access a table with about 120M rows. It's a vertical version
of a table with 360 or so columns. The new columns are: original item col,
original item row, and the value.
I created an index:
CREATE INDEX idx on table (col, row)
however, selects are still very slow. It seems it still needs a sequential
scan:
EXPLAIN SELECT * FROM table WHERE col=1 AND row=10;
QUERY PLAN
------------------------------------------------------------------------------
Seq Scan on table (cost=100000000 .00..102612533. 00 rows=1 width=14)
Filter: ((col = 1) AND ("row" = 10))
What am I doing wrong?
--
Dan Pelleg
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TIP 7: don't forget to increase your free space map settings 6 1644
Scott Marlowe writes: On Wed, 2004-10-20 at 08:06, Dan Pelleg wrote: I'm trying to access a table with about 120M rows. It's a vertical version of a table with 360 or so columns. The new columns are: original item col, original item row, and the value.
I created an index:
CREATE INDEX idx on table (col, row)
however, selects are still very slow. It seems it still needs a sequential scan:
EXPLAIN SELECT * FROM table WHERE col=1 AND row=10; QUERY PLAN ------------------------------------------------------------------------------ Seq Scan on table (cost=100000000 .00..102612533. 00 rows=1 width=14) Filter: ((col = 1) AND ("row" = 10))
What am I doing wrong?
What type are row and col? If they're bigint (i.e. not int / int4) then you might need to quote the value to get the query to use an index:
SELECT * FROM table WHERE col='1' AND row='10';
also, have you vacuumed / analyzed the table? I'm assuming yes.
They're not bigints:
CREATE TABLE table (col int2, row integer, val double precision)
Yes, I vacuumed and analyzed, right after creating the index. Should I try
and issue a few queries beforehand?
--Dan
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TIP 6: Have you searched our list archives? http://archives.postgresql.org
On Wed, 2004-10-20 at 08:06, Dan Pelleg wrote: I'm trying to access a table with about 120M rows. It's a vertical version of a table with 360 or so columns. The new columns are: original item col, original item row, and the value.
I created an index:
CREATE INDEX idx on table (col, row)
however, selects are still very slow. It seems it still needs a sequential scan:
EXPLAIN SELECT * FROM table WHERE col=1 AND row=10; QUERY PLAN ------------------------------------------------------------------------------ Seq Scan on table (cost=100000000 .00..102612533. 00 rows=1 width=14) Filter: ((col = 1) AND ("row" = 10))
What am I doing wrong?
What type are row and col? If they're bigint (i.e. not int / int4) then
you might need to quote the value to get the query to use an index:
SELECT * FROM table WHERE col='1' AND row='10';
also, have you vacuumed / analyzed the table? I'm assuming yes.
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TIP 8: explain analyze is your friend
On Wed, 2004-10-20 at 09:45, Dan Pelleg wrote: Scott Marlowe writes: > On Wed, 2004-10-20 at 08:06, Dan Pelleg wrote: > > I'm trying to access a table with about 120M rows. It's a vertical version > > of a table with 360 or so columns. The new columns are: original item col, > > original item row, and the value. > > > > I created an index: > > > > CREATE INDEX idx on table (col, row) > > > > however, selects are still very slow. It seems it still needs a sequential > > scan: > > > > EXPLAIN SELECT * FROM table WHERE col=1 AND row=10; > > QUERY PLAN > > ------------------------------------------------------------------------------ > > Seq Scan on table (cost=100000000 .00..102612533. 00 rows=1 width=14) > > Filter: ((col = 1) AND ("row" = 10)) > > > > What am I doing wrong? > > What type are row and col? If they're bigint (i.e. not int / int4) then > you might need to quote the value to get the query to use an index: > > SELECT * FROM table WHERE col='1' AND row='10'; > > also, have you vacuumed / analyzed the table? I'm assuming yes.
They're not bigints:
CREATE TABLE table (col int2, row integer, val double precision)
Yes, I vacuumed and analyzed, right after creating the index. Should I try and issue a few queries beforehand?
but one is an int2 (i.e. not int / int4) so you'll need to quote that
value to get an index to work. Note this is fixed in 8.0 I understand.
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TIP 8: explain analyze is your friend
Scott Marlowe writes: On Wed, 2004-10-20 at 09:45, Dan Pelleg wrote: Scott Marlowe writes: > On Wed, 2004-10-20 at 08:06, Dan Pelleg wrote: > > I'm trying to access a table with about 120M rows. It's a vertical version > > of a table with 360 or so columns. The new columns are: original item col, > > original item row, and the value. > > > > I created an index: > > > > CREATE INDEX idx on table (col, row) > > > > however, selects are still very slow. It seems it still needs a sequential > > scan: > > > > EXPLAIN SELECT * FROM table WHERE col=1 AND row=10; > > QUERY PLAN > > ------------------------------------------------------------------------------ > > Seq Scan on table (cost=100000000 .00..102612533. 00 rows=1 width=14) > > Filter: ((col = 1) AND ("row" = 10)) > > > > What am I doing wrong? > > What type are row and col? If they're bigint (i.e. not int / int4) then > you might need to quote the value to get the query to use an index: > > SELECT * FROM table WHERE col='1' AND row='10'; > > also, have you vacuumed / analyzed the table? I'm assuming yes.
They're not bigints:
CREATE TABLE table (col int2, row integer, val double precision)
Yes, I vacuumed and analyzed, right after creating the index. Should I try and issue a few queries beforehand?
but one is an int2 (i.e. not int / int4) so you'll need to quote that value to get an index to work. Note this is fixed in 8.0 I understand.
Bingo.
=> explain select * from table where col='302' and row =100600400;
QUERY PLAN
---------------------------------------------------------------------
Index Scan using idx2 on table (cost=0.00..5.2 7 rows=1 width=14)
Index Cond: ((col = 302::smallint) AND ("row" = 100600400))
(2 rows)
=> explain select * from table where col=302 and row =100600400;
QUERY PLAN
------------------------------------------------------------------------
Seq Scan on table (cost=100000000 .00..102612533. 00 rows=1 width=14)
Filter: ((col = 302) AND ("row" = 100600400))
(2 rows)
Wow, that sure is a big difference for such a small "change" in the
query. Thank you very much!
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
Scott Marlowe wrote: On Wed, 2004-10-20 at 08:06, Dan Pelleg wrote:
I'm trying to access a table with about 120M rows. It's a vertical version of a table with 360 or so columns. The new columns are: original item col, original item row, and the value.
I created an index:
CREATE INDEX idx on table (col, row)
however, selects are still very slow. It seems it still needs a sequential scan:
EXPLAIN SELECT * FROM table WHERE col=1 AND row=10; QUERY PLAN ------------------------------------------------------------------------------ Seq Scan on table (cost=100000000 .00..102612533. 00 rows=1 width=14) Filter: ((col = 1) AND ("row" = 10))
What am I doing wrong?
What type are row and col? If they're bigint (i.e. not int / int4) then you might need to quote the value to get the query to use an index:
SELECT * FROM table WHERE col='1' AND row='10';
also, have you vacuumed / analyzed the table? I'm assuming yes.
I assume not, seen that cost...
Regards
Gaetano Mendola
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Hash: SHA1
Scott Marlowe wrote:
| On Fri, 2004-10-22 at 17:11, Gaetano Mendola wrote:
|
|>Scott Marlowe wrote:
|>
|>>On Wed, 2004-10-20 at 08:06, Dan Pelleg wrote:
|>>
|>>
|>>>I'm trying to access a table with about 120M rows. It's a vertical version
|>>>of a table with 360 or so columns. The new columns are: original item col,
|>>>original item row, and the value.
|>>>
|>>>I created an index:
|>>>
|>>>CREATE INDEX idx on table (col, row)
|>>>
|>>>however, selects are still very slow. It seems it still needs a sequential
|>>>scan:
|>>>
|>>>EXPLAIN SELECT * FROM table WHERE col=1 AND row=10;
|>>> QUERY PLAN
|>>>------------------------------------------------------------------------------
|>>>Seq Scan on table (cost=100000000 .00..102612533. 00 rows=1 width=14)
|>>> Filter: ((col = 1) AND ("row" = 10))
|>>>
|>>>What am I doing wrong?
|>>
|>>
|>>What type are row and col? If they're bigint (i.e. not int / int4) then
|>>you might need to quote the value to get the query to use an index:
|>>
|>>SELECT * FROM table WHERE col='1' AND row='10';
|>>
|>>also, have you vacuumed / analyzed the table? I'm assuming yes.
|>
|>I assume not, seen that cost...
|>
|
|
| Actually, that cost would likely be caused by set enable_seqscan = off
| wouldn't it?
That's true. This is the second time in these last days that I see someone "tune"
postgres setting enable_seqscan = off.
G.
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