Query execution is slow in PL/SQL anonymous block - sql

Question

The query runs slow in Oracle PL/SQL. It takes about 8 minutes in PL/SQL whereas it takes only 4 seconds when run in SQL Editor or SQL Plus.
Please let me know what is causing this. Is it possible that a different execution plan is picked up by SQL and PL/SQL ?
----SQL Editor query---takes 4 seconds---400 row count--
SELECT count(*) FROM
(
SELECT col1, col2
FROM
my_tab1 t1, my_tab2 t2
WHERE
t1.pk_col1=t2.pk_col1
and t1.created_date < t2.created_date
)
--PL/SQL Code--takes about 8 minutes---400 row rount--
DECLARE
v_cnt PLS_INTEGER:=0;
BEGIN
SELECT count(*) INTO v_cnt
FROM
(
SELECT col1, col2
FROM
my_tab1 t1, my_tab2 t2
WHERE
t1.pk_col1=t2.pk_col1
and t1.created_date < t2.created_date
)
END;
/

Answer 1

The easiest way to capture the execution plan within an anonymous block is to call dbms_xplan.display_cursor in a loop and print each line of output:
declare
v_cnt pls_integer;
begin
execute immediate 'alter session set statistics_level = ALL';
select count(*) into v_cnt
from
(
select col1, col2
from my_tab1 t1, my_tab2 t2
where t1.pk_col1 = t2.pk_col1
and t1.created_date < t2.created_date
);
for r in (
select p.plan_table_output
from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST -OUTLINE +NOTE +PREDICATE +IOSTATS +REPORT')) p
)
loop
dbms_output.put_line(r.plan_table_output);
end loop;
end;
You can make the same call from a SQL*Plus command prompt immediately after executing a SQL statement, but you first have to disable dbms_output, as otherwise the SQL statement is not the 'last' statement you made. (You can also specify the sql_id if you know it.) For more details see the dbms_xplan documentation.
set serverout off
alter session set statistics_level = ALL;
select count(*)
from
(
select col1, col2
from my_tab1 t1, my_tab2 t2
where t1.pk_col1 = t2.pk_col1
and t1.created_date < t2.created_date
);
select p.plan_table_output
from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST +OUTLINE +ADAPTIVE +PARTITION +NOTE')) p;
For a handy script to call this in one line, see www.williamrobertson.net/documents/xplanx.html. Then it's just
select count(*)
from
(
select col1, col2
from my_tab1 t1, my_tab2 t2
where t1.pk_col1 = t2.pk_col1
and t1.created_date < t2.created_date
)
#xplanx