Re: EXECUTE 'EXECUTE ...' or how to use dynamic prepared statements ?
Re: EXECUTE 'EXECUTE ...' or how to use dynamic prepared statements ?
От:
"Dirk Jagdmann" <jagdmann@gmail.com>
Дата:
Hello Marc, at first I tried to solve your update of the tables. The example you gave should be done with an update statement like the following: update test_table set mygroup=(select t.mygroup from test_table as t where t.family = test_table.family and t.rang = test_table.rang+1)where rang=0; If you have to write a function which receives the tablename as an argument it would look like: CREATE OR REPLACE FUNCTION test_function(tablename text) RETURNS integer AS $BODY$BEGIN EXECUTE 'update ' || tablename || ' set mygroup=(select t.mygroup from ' || tablename || ' as t where t.family = test_table.family and t.rang = test_table.rang+1)where rang=0;' RETURN 0;END; $BODY$ LANGUAGE 'plpgsql' VOLATILE; Generally you should avoid using explicit for/loop constructs in your stored procedures if the action can be solved by a single SQL statement, because the optimizer can make a better execution plan. -- ---> Dirk Jagdmann ----> http://cubic.org/~doj -----> http://llg.cubic.org
EXECUTE 'EXECUTE ...' or how to use dynamic prepared statements ?
От:
"Marc Mamin" <M.Mamin@intershop.de>
Дата:
Hello,
I have a large upddate to perform on tables which are dynamically
generated (dynamic names).
In this simplified example, the operation should replace in each family
the "mygroup" of each item of rang=0 with the "mygroup" value of the
element of rang=1 :
(the * indicate the modified values)
id family rang mygroup
1 1 0 1
2 1 1 2
3 1 2 3
4 1 3 4
5 2 0 6
6 2 1 6
7 2 2 7
8 2 3 7
9 3 0 10
10 3 1 20
11 3 2 21
After the update:
1 1 0 2 *
2 1 1 2
3 1 2 3
4 1 3 4
5 2 0 6
6 2 1 6
7 2 2 7
8 2 3 7
9 3 0 20 *
10 3 1 20
11 3 2 21
In the following function, I would like to use a prepared statement for
the update command but I get stuck with the tho different meanings of
EXECUTE ...
Is there a way to achieve this ?
Thanks,
Marc
CREATE OR REPLACE FUNCTION test_function(tablename varchar) RETURNS integer AS
$BODY$
DECLARE
rec record;
top_group int;
top_family character(16);
top_id int;
BEGIN /* the prepared statement must be generated dynamically in order to
include the table name. */ EXECUTE 'PREPARE update_stmt (int, int) AS update '||tablename||' set mygroup= $1 where id = $2';
/* using "select distinct on" allows to retrieve and sort the required
information for the update. this is faster than a self join on the table */ for rec in execute 'select DISTINCT on (family,rang) family, rang, mygroup, id from '||tablename||' where rang < 2 order by family, rang' loop IF rec.rang = 0 THEN top_group := rec.mygroup; top_family := rec.family; top_id := rec.id; ELSIF rec.family = top_family AND rec.mygroup <> top_group THEN /* Update without using a prepared statement EXECUTE 'update '||tablename||' set mygroup=
'||rec.mygroup||' where id = '||top_id; */ -- This works, but the command has to be computed for each
iteration EXECUTE 'EXECUTE
update_stmt('||rec.mygroup||','||top_id||')'; /* Following syntax would be fine PERFORM EXECUTE update_stmt(rec.mygroup,top_id); */ END IF;
end loop; DEALLOCATE update_stmt; RETURN 0;
END;
$BODY$ LANGUAGE 'plpgsql' VOLATILE;
/* ============================================
test data:
=============================================== */
--drop table test_table;
create table test_table(id int,family int,rang int,mygroup int);
insert into test_table values (1,1,0,1);
insert into test_table values (2,1,1,2);
insert into test_table values (3,1,2,3);
insert into test_table values (4,1,3,4);
insert into test_table values (5,2,0,6);
insert into test_table values (6,2,1,6);
insert into test_table values (7,2,2,7);
insert into test_table values (8,2,3,7);
insert into test_table values (9, 3,0,10);
insert into test_table values (10,3,1,20);
insert into test_table values (11,3,2,21);
select test_function('test_table');
select * from test_table order by id;
Re: EXECUTE 'EXECUTE ...' or how to use dynamic prepared statements ?
От:
"Marc Mamin" <M.Mamin@intershop.de>
Дата:
Hello Dirk, I have to disagree. Your first update query is very low. It probably implies to run the sub select statement for each row to be updated. Following update statement is already much faster: (using UPDATE FROM) update test_table set mygroup= t.mygroup from test_table as t where t.family = test_table.family and t.rang = 1 and table.rang=0 -- perform the updte only when required and mygroup <> t.mygroup; But when you are dealing with "parent - child" relations within a single table as in my case, a single table scan with SELECT DISTINCT ON and a row by row comparison on the result set appears to be faster. I tested both approaches on tables with ca. 14'000'000 rows where 25% of them needed to be updated. The above update statement run in 5H30' where my function did the job in 2H. (as my tables are very large, much time is lost in i/o wait) Cheers, Marc
