hello all ...
given oleg's posting before i also wanted to fire up some KNN related question.
let us consider a simple example. i got some million lines and i want all rows
matching a tsquery sorted by price.
i did some tests:
test=# explain (analyze true, buffers true, costs true) SELECT id FROM
product.t_product WHERE to_tsvector('german', title) @@ to_tsquery('german',
'iphone') ORDER BY int_price <-> 0 LIMIT 10;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
--------------------------------------
Limit (cost=0.00..41.11 rows=10 width=16) (actual time=36391.717..45542.590
rows=10 loops=1)
Buffers: shared hit=9 read=5004
-> Index Scan using idx_product_t_product_titleprice on t_product
(cost=0.00..13251.91 rows=3224 width=16) (actual time=
36391.715..45542.573 rows=10 loops=1)
Index Cond: (to_tsvector('german'::regconfig, title) @@
'''iphon'''::tsquery)
Order By: (int_price <-> 0::bigint)
Buffers: shared hit=9 read=5004
Total runtime: 45542.676 ms
(7 rows)
test=# explain (analyze true, buffers true, costs true) SELECT id FROM
product.t_product WHERE to_tsvector('german', title) @@ to_tsquery('german',
'handy') ORDER BY int_price <-> 0 LIMIT 10;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
-------------------------------------
Limit (cost=0.00..41.03 rows=10 width=16) (actual time=7243.526..10935.227
rows=10 loops=1)
Buffers: shared hit=3 read=2316
-> Index Scan using idx_product_t_product_titleprice on t_product
(cost=0.00..29762.61 rows=7255 width=16) (actual time=
7243.524..10935.217 rows=10 loops=1)
Index Cond: (to_tsvector('german'::regconfig, title) @@
'''handy'''::tsquery)
Order By: (int_price <-> 0::bigint)
Buffers: shared hit=3 read=2316
Total runtime: 10935.265 ms
(7 rows)
test=# explain (analyze true, buffers true, costs true) SELECT id FROM
product.t_product WHERE to_tsvector('german', title) @@ to_tsquery('german',
'handy') ORDER BY int_price <-> 0 LIMIT 1;
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------------------
-------------------------------
Limit (cost=0.00..4.10 rows=1 width=16) (actual time=28.527..28.528 rows=1
loops=1)
Buffers: shared hit=1 read=1577
-> Index Scan using idx_product_t_product_titleprice on t_product
(cost=0.00..29762.61 rows=7255 width=16) (actual time=
28.525..28.525 rows=1 loops=1)
Index Cond: (to_tsvector('german'::regconfig, title) @@
'''handy'''::tsquery)
Order By: (int_price <-> 0::bigint)
Buffers: shared hit=1 read=1577
Total runtime: 28.558 ms
(7 rows)
under any circumstances - there is no way to reduce the number of buffers
needed for a query like that.
if everything is cached this is still ok but as soon as you have to take a
single block from disk you will die a painful random I/O death.
is there any alternative which does not simply die when i try to achieve what i
want?
the use case is quite simple: all products with a certain word (10 cheapest or
so).
is there any alternative approach to this?
i was putting some hope into KNN but it seems it needs too much random I/O :(.
many thanks,
hans
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