Is it possible to do this :
CREATE TABLE sorted (order_no SERIAL PRIMARY KEY, other columns...)
INSERT INTO sorted (columns) SELECT * FROM main_table INNER JOIN
key_table ON main_table.id = key_table.main_table_id WHERE key = 'param'
ORDER BY value SELECT
The SERIAL will automatically generate the order_no you want, which
corresponds to the position in the sorted set.
Then, to get the records in-order :
SELECT * FROM sorted ORDER BY order_no
As the records have been inserted in-order in the "sorted" table, this
table is, in fact, clustered, so a full table scan using the index on
"order_no" will be very fast.
Of course this is only interesting if this data is quite static, because
you'll have to re-generate the table when the data changes.
There is another solution :
CREATE INDEX on key_table( key, value )
Now, the index can optimize ordering by (key,value), which is equivalent
to ordering by value if key = constant. A bit of query manipulation might
get you what you want ; I suppose all rows in "key_table" reference a row
in "main_table" ; so it is faster to sort (and limit) first on key_table,
then grab the rows from main_table :
SELECT k.value, m.* FROM key_table k LEFT JOIN main_table m ON
m.id=k.main_table_id WHERE k.key='param' ORDER BY k.key, k.value
If key_table REFERENCES main_table, LEFT JOIN is equivalent to INNER JOIN
; however if the planner is smart enough, it might notice that it can
index-scan key_table in key,value order, grabbing rows from main_table in
order and skip the sort entirely.
On Sun, 07 May 2006 08:53:46 +0200, Ben K. <[EMAIL PROTECTED]> wrote:
main_table: id, name, position
key_table: id, main_table_id, key, value
Here is how I need to sort the records:
SELECT * FROM main_table
INNER JOIN key_table ON main_table.id = key_table.main_table_id
WHERE key = 'param'
ORDER BY value
I currently collect all ids from main_table in sorted order and then
update the position field for each row in the main_table one-by-one. Is
there a better/faster/more efficient solution?
A cheap solution if you don't care about the position value as long as
sort order is ok.
1)
# SELECT main_table.id into temp_table FROM main_table INNER JOIN
key_table ON main_table.id = key_table.main_table_id ORDER BY value;
2)
# update main_table set position = (select oid from temp_table where id
= main_table.id );
I guess I'll get a set of consecutive oids by this.
You can make the number begin at arbitrary number, by
2-a)
# update main_table set position = ( (select oid::int4 from temp_table
where id = main_table.id ) - (select min(oid::int4) from temp_table)
+ 1) ;
I read that oid wraps around (after ~ billions) so you might want to
check your current oid.
Regards,
Ben K.
Developer
http://benix.tamu.edu
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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
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