I ran into a situation today maintaining someone else's code where the sum time running 2 queries seems to be faster than 1. The original code was split into two queries. I thought about joining them, but considering the intelligence of my predecessor, I wanted to test it.
The question is, which technique is really faster? Is there some hidden setup cost I don't see with explain analyze?
Postgres 7.4.7, Redhat AES 3
Each query individually:
test=> explain analyze test-> select * from order WHERE ord_batch='343B' AND ord_id='12-645'; QUERY PLAN ---------------------------------------------------------------------------------------------------------------------------- Index Scan using order_pkey on order (cost=0.00..6.02 rows=1 width=486) (actual time= 0.063..0.066 rows=1 loops=1) Index Cond: ((ord_batch = '343B'::bpchar) AND (ord_id = '12-645'::bpchar)) Total runtime: 0.172 ms (3 rows)
test=> explain analyze test-> select cli_name from client where cli_code='1837'; QUERY PLAN --------------------------------------------------------------------------------------------------------------------- Index Scan using client_pkey on client (cost=0.00..5.98 rows=2 width=39) (actual time= 0.043..0.047 rows=1 loops=1) Index Cond: (cli_code = '1837'::bpchar) Total runtime: 0.112 ms (3 rows)
Joined:
test=> explain analyze test-> SELECT cli_name,order.* test-> FROM order test-> JOIN client ON (ord_client = cli_code) test-> WHERE ord_batch='343B' AND ord_id='12-645';
where is the cli_code condition in the above query?
QUERY PLAN ---------------------------------------------------------------------------------------------------------------------------------- Nested Loop (cost=0.00..12.00 rows=2 width=525) (actual time=0.120..0.128 rows=1 loops=1) -> Index Scan using order_pkey on order (cost=0.00..6.02 rows=1 width=486) (actual time=0.064..0.066 rows=1 loops=1) Index Cond: ((ord_batch = '343B'::bpchar) AND (ord_id = '12-645'::bpchar)) -> Index Scan using client_pkey on client (cost=0.00..5.98 rows=1 width=51) (actual time=0.023..0.026 rows=1 loops=1) Index Cond: ("outer".ord_client = client.cli_code) Total runtime: 0.328 ms (6 rows)
