locking problem

hi,

can anyone help me out on the following scenario:
why this is happening, if i'm doing any thing wrong or its the feature of postgres...

regards
cheetor
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PostgreSQL
Steps:

1. Create a table
create table mytab (name varchar(100), marks NUMERIC(9));

2. insert a row into the table:
INSERT INTO mytab (name, marks) VALUES ('abc', 3);

3. compile the function myproc (at end of mail)

4. Open sql prompt and type:
begin;
select myproc(1, 'xyz', 3, 'abc', 10, 'pqr');

This would insert into the table the values 'xyz' and 1.

5. Open another sql prompt and type:
begin;
select myproc(10, 'pqr', 3, 'abc', 1, 'xyz');

This would try and insert into the table values 'pqr' and 10.

But as the query in step4 has locked the table records, the query of
step 5 would wait..

6. On the first sql prompt type commit;
This would let the transaction of step 5 complete, but it outputs the
statement "not exists". This means that even after the transaction was
commited, the insert of step 4 was not visible in query of step 5.

7. on sql prompt of step 5, again type
select myproc(10, 'pqr', 3, 'abc', 1, 'xyz');

and this outputs "exists" which means that now the insert is visible.
Therefore it implies that if the second transaction is blocking on a
locked resource, after it resumes, it does not see any inserts, but if
has not blocked, these inserts are visible.

The same steps were tried on oracle 8.1.7.
Steps:

1. Create a table
create table mytab (name varchar(100), marks int);

2. insert a row into the table:
INSERT INTO mytab (name, marks) VALUES ('abc', 3);
commit;

3. compile the procedure myproc (at end of mail)

4. Open sql prompt (set server output on) and type:
exec myproc(1, 'xyz', 2, 'abc', 10, 'pqr');

5. Open another sql prompt and type (set server output on):
exec myproc(10, 'pqr', 2, 'abc', 1, 'xyz');
But as the query is step4 has locked the table records, the query of
step 5 would wait..

6. On the first sql type commit;
This would let the transaction of step 5 complete, and it outputs the
statement "exists". This means that after the transaction was
commited, the insert of step 4 is visible in query of step 5.

______________________________________________________________________
Postgres function
____________________________________________________________________

CREATE FUNCTION myproc (INT8, VARCHAR, INT8, VARCHAR, INT8, VARCHAR)
RETURNS TEXT AS '
DECLARE

DBMarks ALIAS FOR $1;
DBName ALIAS FOR $2;

DBMarks2 ALIAS FOR $3;
DBName2 ALIAS FOR $4;

DBMarks3 ALIAS FOR $5;
DBName3 ALIAS FOR $6;

DBMarks4 INT8;
DBName4 VARCHAR (100);

BEGIN

SELECT name, marks
INTO DBName4, DBMarks4
FROM mytab
WHERE name = DBName2
AND marks = DBMarks2 FOR UPDATE;

raise notice '' name : % : marks : % :'', DBName4, DBMarks4;

INSERT INTO mytab (name, marks) VALUES (DBName, DBMarks);

raise notice ''insert done'';

IF EXISTS(SELECT * FROM mytab WHERE name = DBName3 AND marks =
DBMarks3)
THEN
raise notice ''exists'';
ELSE
raise notice ''not exists'';
END IF;

return ''done'';

END;
' language 'plpgsql';

________________________________________________________________________
Oracle procedure
_______________________________________________________________________

CREATE OR REPLACE PROCEDURE myproc
(
DBMarks INT,
DBName VARCHAR,
DBMarks2 INT,
DBName2 VARCHAR,
DBMarks3 INT,
DBName3 VARCHAR
)
AS
DBMarks4 INT;
DBName4 VARCHAR (100);

BEGIN
SELECT name, marks
INTO DBName4, DBMarks4
FROM mytab
WHERE name = DBName2
AND marks = DBMarks2 FOR UPDATE;

dbms_output.put_line(' Name :' || DBName4 || ' : Marks : ' ||
DBMarks4 ||':');

INSERT INTO mytab (name, marks) VALUES (DBName, DBMarks);

dbms_output.put_line('Insert Done');

BEGIN
SELECT name, marks into DBName4, DBMarks4 FROM mytab WHERE name =
DBName3 AND marks = DBMarks3;
dbms_output.put_line('exists');
EXCEPTION
WHEN NO_DATA_FOUND THEN
dbms_output.put_line('not exists');
END;
dbms_output.put_line('done');
END;
________________________________________________________________________