On 5/5/21 3:23 PM, Pavel Luzanov wrote:
Hello,
It is very likely that the date_trunc function in the following example
is executed for each line of the query. Although it marked as a STABLE
and could only be called once.
It could, but that's just an option - the database may do that, but it's
not required to do it. In this case it might be beneficial, but it'd
make the planner more complex etc.
EXPLAIN (ANALYZE)
SELECT * FROM generate_series('2021-01-01', '2021-06-01', '1
s'::interval) AS g(x) WHERE g.x >= date_trunc('day',
'2021-05-05'::timestamptz);
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Function Scan on generate_series g (cost=0.00..15.00 rows=333
width=8) (actual time=2801.884..3263.328 rows=2332801 loops=1)
Filter: (x >= date_trunc('day'::text, '2021-05-05
00:00:00+03'::timestamp with time zone))
Rows Removed by Filter: 10713600
Planning Time: 0.040 ms
Execution Time: 3336.657 ms
When replacing date_trunc with now, the query is much faster:
EXPLAIN (ANALYZE)
SELECT * FROM generate_series('2021-01-01', '2021-06-01', '1
s'::interval) AS g(x)
WHERE g.x >= now();
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Function Scan on generate_series g (cost=0.00..15.00 rows=333
width=8) (actual time=1648.777..1845.430 rows=2275325 loops=1)
Filter: (x >= now())
Rows Removed by Filter: 10771076
Planning Time: 0.039 ms
Execution Time: 1918.767 ms
The variant with now works almost as fast as with the constant. This
suggests me that perhaps date_trunc is being executed for every line of
the query:
EXPLAIN (ANALYZE)
SELECT * FROM generate_series('2021-01-01', '2021-06-01', '1
s'::interval) AS g(x)
WHERE g.x >= '2021-05-05'::timestamptz;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Function Scan on generate_series g (cost=0.00..12.50 rows=333
width=8) (actual time=1628.743..1826.841 rows=2332801 loops=1)
Filter: (x >= '2021-05-05 00:00:00+03'::timestamp with time zone)
Rows Removed by Filter: 10713600
Planning Time: 0.033 ms
Execution Time: 1901.680 ms
In this regard, I have two questions:
1. How can I find out exactly how many times the date_trunc function has
been executed? So far, these are just my assumptions.
2. If date_trunc is indeed called multiple times, why is this happening?
Well, it'd not like date_trunc is executed for each row while now() is
executed only once. The functions are executed for each row in both
cases, but now() is simply much cheaper - it just returns a value that
is already calculated, while date_trunc has to parse and truncate the
value, etc.
You can use CTE to execute it just once, I think:
with x as (select date_trunc('day', '2021-04-01'::timestamptz) as x)
select * from t where a > (select x from x);
regards
Tomas
