Re: Sorting on longer key is faster ?

jobapply wrote:
> The 2 queries are almost same, but ORDER BY x||t is FASTER than ORDER BY x..
>
> How can that be possible?
>
> Btw: x and x||t are same ordered
>
> phoeniks=> explain analyze SELECT * FROM test WHERE i<20 ORDER BY x || t;
> QUERY PLAN
>

What types are x and t, I have the feeling "x || t" is actually a
boolean, so it is only a True/False sort, while ORDER BY x has to do
some sort of string comparison (which might actually be a locale
depended comparison, and strcoll can be very slow on some locales)

John
=:->

> ----------------------------------------------------------------------------
> ----------------------------------------------
> Sort (cost=2282.65..2284.92 rows=907 width=946) (actual
> time=74.982..79.114 rows=950 loops=1)
> Sort Key: (x || t)
> -> Index Scan using i_i on test (cost=0.00..2238.09 rows=907 width=946)
> (actual time=0.077..51.015 rows=950 loops=1)
> Index Cond: (i < 20)
> Total runtime: 85.944 ms
> (5 rows)
>
> phoeniks=> explain analyze SELECT * FROM test WHERE i<20 ORDER BY x;
> QUERY PLAN
> ----------------------------------------------------------------------------
> ---------------------------------------------
> Sort (cost=2280.38..2282.65 rows=907 width=946) (actual
> time=175.431..179.239 rows=950 loops=1)
> Sort Key: x
> -> Index Scan using i_i on test (cost=0.00..2235.82 rows=907 width=946)
> (actual time=0.024..5.378 rows=950 loops=1)
> Index Cond: (i < 20)
> Total runtime: 183.317 ms
> (5 rows)
>
>
>
>
>
> phoeniks=> \d+ test
> Table "public.test"
> Column | Type | Modifiers | Description
> --------+---------+-----------+-------------
> i | integer | |
> t | text | |
> x | text | |
> Indexes:
> "i_i" btree (i)
> "x_i" btree (xpath_string(x, 'data'::text))
> "x_ii" btree (xpath_string(x, 'movie/characters/character'::text))
> Has OIDs: no
>
>
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