temporary table vs array performance

Its considered bad form to post to multiple lists. Please pick the most
relevant one - in this case I'd suggest -general.

On Mon, Sep 26, 2016 at 8:39 AM, dbyzaa@163.com <dbyzaa@163.com> wrote:

Array is not convenient to use in function, whether
there are other methods can be replaced temp table in function

​I have no difficulty using arrays in functions.

As for "other methods" - you can use CTE (WITH) to create a truly local
table - updating the catalogs by using a temp table is indeed quite
expensive.

WITH vals AS ( VALUES (1, 'lw'), (2, 'lw2') )
SELECT * FROM vals;

David J.

2016-09-26 17:39 GMT+02:00 dbyzaa@163.com <dbyzaa@163.com>:

test:
create type h3 as (id int,name char(10));

CREATE or replace FUNCTION proc17()
RETURNS SETOF h3 AS $$
DECLARE
v_rec h3;
BEGIN
create temp table abc(id int,name varchar) on commit drop;
insert into abc select 1,'lw';
insert into abc select 2,'lw2';
for v_rec in
select * from abc loop
return next v_rec;
end loop;
END;
$$
LANGUAGE plpgsql;

CREATE or replace FUNCTION proc16()
RETURNS SETOF h3 AS $$
DECLARE
id_array int[];
name_arr varchar[];
v_rec h3;
BEGIN
id_array =array[1,2];
name_arr=array['lw','lw2'];
for v_rec in
select unnest(id_array) ,unnest(name_arr) loop
return next v_rec;
end loop;
END;
$$
LANGUAGE plpgsql;
postgres=# select * from proc17();
id | name
----+------------
1 | lw
2 | lw2
(2 rows)

Time: 68.372 ms
postgres=# select * from proc16();
id | name
----+------------
1 | lw
2 | lw2
(2 rows)

Time: 1.357 ms

temp talbe result:
[postgres@pg95 test_sql]$ pgbench -M prepared -n -r -c
2 -j 2 -T 10 -f temporary_test_1.sql
transaction type: Custom query
scaling factor: 1
query mode: prepared
number of clients: 2
number of threads: 2
duration: 10 s
number of transactions actually processed: 5173
latency average: 3.866 ms
tps = 517.229191 (including connections establishing)
tps = 517.367956 (excluding connections establishing)
statement latencies in milliseconds:
3.863798 select * from proc17();

array result:
[postgres@pg95 test_sql]$ pgbench -M prepared -n -r -c
2 -j 2 -T 10 -f arrary_test_1.sql
transaction type: Custom query
scaling factor: 1
query mode: prepared
number of clients: 2
number of threads: 2
duration: 10 s
number of transactions actually processed: 149381
latency average: 0.134 ms
tps = 14936.875176 (including connections establishing)
tps = 14940.234960 (excluding connections establishing)
statement latencies in milliseconds:
0.132983 select * from proc16();

Array is not convenient to use in function, whether
there are other methods can be replaced temp table in function

Temporary tables are pretty expensive - from more reasons, and horrible
when you use fresh table for two rows only. More if you recreate it every
transaction.

More often pattern is create first and delete repeatedly. Better don't use
temp tables when it is necessary. It is one reason why PostgreSQL supports
a arrays. Partially - PostgreSQL arrays are analogy to T-SQL memory tables.

Regards

Pavel

On Mon, Sep 26, 2016 at 9:18 AM, 邓彪 <dbyzaa@163.com> wrote:

we have to do dml in temp table,the CTE is not fit

​Moving this to -general only...​

​Please direct all replies to the list.

You are asking for help but not providing any context for what your
requirements are. You are not likely to get good help.

Best case, supply a working function (self contained test case) that does
exactly what you need it to do but uses a temporary table and performs
badly. Lacking that at least attempt to describe your problem and not just
point out that creating temporary tables is expensive.

David J.
​