here is another algorithm using cursor but i'm having a hard time fixing its error ...
CREATE OR REPLACE FUNCTION extractstudent()
RETURNS VOID AS
$BODY$
DECLARE
studcur SCROLL cursor FOR SELECT fname, lname, mname, address FROM student;
BEGIN
open studcur;
Loop
--fetching 1 row at a time
FETCH First FROM studcur;
--every row fetched is being inserted to another database on the local site
--myconT is the name of the connection to the other database in the local site
execute 'SELECT * from dblink_exec(''myconT'', ''insert into temp_student values(studcur)'')';
--move to the next row and execute again
move next from studcur;
--exit when the row content is already empty
exit when studcur is null;
end loop;
close studcur;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION extractstudent() OWNER TO postgres;
You rarely need to explicitly use cursors in postgresql or pl/pgsql. What you've written looks suspiciously like a SQL Server cursor loop construct, and you don't need to do that. Also, you can use "PERFORM" instead of "EXECUTE" to run a query and discard the results: this will avoid re-parsing the query each time (although it can't avoid dblink parsing the query each time).
You can do something more like this:
DECLARE
rec student%rowtype;
BEGIN
FOR rec IN SELECT * FROM student
LOOP
PERFORM dblink_exec('myconT',
'insert into temp_student values ('
|| quote_nullable(rec.fname) || ','
|| quote_nullable(rec.lname) || ','
|| quote_nullable(rec.mname) || ','
|| quote_nullable(rec.address) || ')');
END LOOP;
END;
Why not try it by yourself , according the error, you can try to solve them step by step !
Related
I am very new to DB2 even though have experience in Oracle. I am not able to resolve this issue.I have a requirement where I need to find missing child records in the parent table .The parent table , child table and the join_key are all passed as input parameter.
I have tried this in a procedure was able to achieve this, but the admin wants it in a function so that they can just use it in a select statment and get the result in a table format. Since the parent table , child table and the join_key are comming as input parement, I am not able to run them as dynamic sql.
create or replace function missing_child_rec(PARENT_TBL VARCHAR(255),JOIN_KEY VARCHAR(255),CHILD_TBL VARCHAR(255))
RETURNS TABLE(Key VARCHAR(255))
LANGUAGE SQL
BEGIN
DECLARE V_SQL VARCHAR(500);
DECLARE C_SQL CURSOR WITH RETURN FOR S_SQL;
SET V_PARENT_TAB = PARENT_TBL;
SET V_KEY = JOIN_KEY;
SET V_CHILD_TAB = CHILD_TBL;
SET V_SQL = 'SELECT DISTINCT '|| JOIN_KEY || ' FROM ' || V_CHILD_TAB || ' A WHERE NOT EXISTS
(SELECT ' ||V_KEY || ' FROM ' || V_PARENT_TAB || ' B WHERE A.'||JOIN_KEY || '= B.'||JOIN_KEY ||' )' ;
PREPARE S_SQL FROM V_SQL;
OPEN C_SQL;
CLOSE C_SQL;
RETURN
END
When I try to compile it , it says prepare is invalid , I have tried even execute immediate but that also gave error.Can you please help me with how to use dynamic sql in UDF or an alternative logic for this problem
There is more than one way to solve this, here's one way.
If you already have a working stored-procedure that returns the correct result-set then you can call that stored-procedure from a pipelined table function. The idea is that a pipelined table function can consume the result-set and pipe it to the caller.
This will work on Db2-LUW v10.1 or higher, as long as the database is not partitioned over multiple nodes.
It may work on Db2-for-i v7.1 or higher.
It will not work with Db2 for Z/os at current versions.
Suppose your stored procedure is sp_missing_child_rec and it takes the same input parameters as the function you show in your question, and suppose the data type of the join column is varchar(100).
The pipelined wrapper table function would look something like this:
--#SET TERMINATOR #
create or replace function missing_child_rec(PARENT_TBL VARCHAR(255),JOIN_KEY VARCHAR(255),CHILD_TBL VARCHAR(255))
returns table ( join_column_value varchar(100))
begin
declare v_rs result_set_locator varying;
declare v_row varchar(100); -- to match the join_column_datatype, adjust as necessary
declare sqlstate char(5) default '00000';
CALL sp_missing_child_rec( parent_tbl, join_key, child_tbl);
associate result set locator (v_rs) with procedure sp_missing_child_rec ;
allocate v_rscur cursor for result set v_rs;
fetch from v_rscur into v_row;
while ( sqlstate = '00000') do
pipe(v_row);
fetch from v_rscur into v_row;
end while;
return;
end#
select * from table(missing_child_rec( 'parent_table' , 'join_column', 'child_table'))
#
When initializing my process, it runs the PL/pgSQL statement below creating two functions. However, every time I create multiple processes simultaneously as part of an end-to-end test, parallel execution of this statement leads to a tuple concurrently updated error that I can't seem to get around. Any help would be much appreciated.
CREATE OR REPLACE FUNCTION
count_rows(schema text, tablename text) returns integer
AS
$body$
DECLARE
result integer;
query varchar;
BEGIN
query := 'SELECT count(1) FROM "' || schema || '"."' || tablename || '"';
execute query into result;
return result;
END;
$body$
LANGUAGE plpgsql;
CREATE OR REPLACE FUNCTION
delete_if_empty(schema text, tablename text) RETURNS INTEGER
AS
$$
DECLARE
result integer;
query varchar;
BEGIN
query := 'SELECT COUNT(*) FROM "' || schema || '"."' || tablename || '"';
execute query into result;
IF result = 0 THEN
EXECUTE 'DROP TABLE "' || schema || '"."' || tablename || '" CASCADE;';
EXECUTE 'NOTIFY "' || schema || '", ''DESTROY_TABLE:' || tablename || ''';';
RETURN 1;
END IF;
RETURN 0;
END;
$$
LANGUAGE plpgsql;
SELECT version()
As described here, postgres doesn't currently allow you to use CREATE FUNCTION concurrently:
It'd be necessary to add
some kind of locking scheme if you want to avoid "tuple concurrently
updated" errors. This is not really any different from the situation
where two transactions both want to update the same row in a user table:
unless the application takes extra steps to serialize the updates, you're
going to get "tuple concurrently updated" errors.
We do have such locking for DDL on tables/indexes, but the theory in the
past has been that it's not worth the trouble for objects represented by
single catalog rows, such as functions or roles.
A solution to this is to ensure that no two transaction try to do the CREATE FUNCTION at the same time.
You can use posgres advisory locks for that.
A good introduction to advisory locks can be found here: https://vladmihalcea.com/how-do-postgresql-advisory-locks-work/
For example, you can use:
BEGIN; -- start of transaction
SELECT pg_advisory_xact_lock(2142616474639426746); -- random 64-bit signed ('bigint') lock number
CREATE OR REPLACE FUNCTION myfunction ...
COMMIT;
This takes a transaction-level exclusive advisory lock, so that no two concurrent transaction can run create the function at the same time. At the end of the transaction, the lock is automatically released.
For some fancy database maintenance for my developer database I'd like to be able to use queries to generate commands to alter the database. The thing is: I'm a complete greenhorn to PostgreSQL. I've made my attempt but have failed colorfully.
So in the end, I would like to have a table with a single column and each row would be a command (or group of commands, depending on the case) that I would think would look something like this...
DO $$
DECLARE
command_entry RECORD;
BEGIN
FOR command_entry IN SELECT * FROM list_of_commands
LOOP
EXECUTE command_entry;
END LOOP;
END;
$$;
Where the table list_of_commands could be populated with something like the following (which in this example would remove all tables from the public schema)...
CREATE TEMP TABLE list_of_commands AS
SELECT 'drop table if exists "' || tablename || '" cascade;'
FROM pg_tables
WHERE schemaname = 'public';
However, with this I get the following error...
ERROR: syntax error at or near ""drop table if exists ""dummy_table"" cascade;""
LINE 1: ("drop table if exists ""dummy_table"" cascade;")
I assume this is a matter of escaping characters, but I'm not entirely sure how to fit that into either A) the population of the table or B) the execution of each row. Does anyone know what I could do to achieve the desired result?
The command_entry variable is of type record while the EXECUTE command expects a string. What is apparently happening is that PostgreSQL turns the record into a double-quoted string, but that messes up your command. Also, your temp table does not use a column name, making things a bit awkward to work with (the column name becomes ?column?), so change both as follows:
CREATE TEMP TABLE list_of_commands AS
SELECT 'drop table if exists public.' || quote_ident(tablename) || ' cascade' AS cmd
FROM pg_tables
WHERE schemaname = 'public';
DO $$
DECLARE
command_entry varchar;
BEGIN
FOR command_entry IN SELECT cmd FROM list_of_commands
LOOP
EXECUTE command_entry;
END LOOP;
END;
$$;
But seeing that you do all of this at session level (temp table, anonymous code block), why not write a stored procedure that performs all of this housekeeping when you are ready to do spring cleaning?
CREATE FUNCTION cleanup() RETURNS void AS $$
BEGIN
FOR tbl IN SELECT tablename FROM pg_tables WHERE schemaname = 'public'
LOOP
EXECUTE 'DROP TABLE IF EXISTS ' || quote_ident(tbl) || ' CASCADE';
END LOOP;
-- More housekeeping jobs
END;
$$ LANGUAGE plpgsql;
This saves a lot of typing: SELECT cleanup();. Any other housekeeping jobs you have you simply add to the stored procedure.
I had trouble with Patrick's answers, so here is an updated version for postgreSQL 10.
CREATE FUNCTION droptables(sn varchar) RETURNS void AS $$
DECLARE
tbl varchar;
BEGIN
FOR tbl IN SELECT tablename FROM pg_tables WHERE schemaname = sn
LOOP
EXECUTE 'DROP TABLE IF EXISTS ' || quote_ident(tbl) || ' CASCADE';
END LOOP;
END;
$$ LANGUAGE plpgsql;
And then "SELECT droptables('public');".
In continuation to a previous case (a solution by #Erwin Brandstetter), in which a dynamic SELECT query that uses a 'prepare' statement was created and then was executed by calling it, as below:
--the function for making the prepare statement:
CREATE OR REPLACE FUNCTION f_prep_query (_tbl regclass, _prefix text)
RETURNS void AS
$func$
DECLARE
_prep_qry text := (
SELECT 'PREPARE stmt_dyn AS SELECT '
|| string_agg(quote_ident(attname), ',' ORDER BY attname)
|| ' FROM ' || _tbl
FROM pg_attribute
WHERE attrelid = _tbl
AND attname LIKE _prefix || '%'
AND attnum > 0
AND NOT attisdropped
);
BEGIN
EXECUTE _prep_qry;
EXCEPTION WHEN duplicate_prepared_statement THEN
DEALLOCATE stmt_dyn;
EXECUTE _prep_qry;
END
$func$ LANGUAGE plpgsql;
--the calling:
BEGIN; -- optional
SELECT f_prep_query('dkj_p_k27ac'::regclass, 'enri'::text);
EXECUTE stmt_dyn;
I would like to ask the following:
The desired output that we get from the indicated procedure is outputted into the DataOutput.
I would like to find a way to store the data into a new table in the db.
Generally, if you just want to write to a table, don't use a prepared SELECT statement (or a cursor). That's very inefficient for the purpose.
Write to the table directly like explained in the previous answer:
Saving the output of a dynamic query that uses refcursor into a table
The complete INSERT could be the prepared statement. But not CREATE TABLE AS. Per documentation:
Any SELECT, INSERT, UPDATE, DELETE, or VALUES statement.
I am adjusting some PL/pgSQL code so my refcursor can take the table name as parameter. Therefore I changed the following line:
declare
pointCurs CURSOR FOR SELECT * from tableName for update;
with this one:
OPEN pointCurs FOR execute 'SELECT * FROM ' || quote_ident(tableName) for update;
I adjusted the loop, and voilà, the loop went through. Now at some point in the loop I needed to update the record (pointed by the cursor) and I got stuck. How should I properly adjust the following line of code?
UPDATE tableName set tp_id = pos where current of pointCurs;
I fixed the quotes for the tableName and pos and added the EXECUTE clause at the beginning, but I get the error on the where current of pointCurs.
Questions:
How can I update the record?
The function was working properly for tables from the public schema and failed for tables from other schemas (e.g., trace.myname).
Any comments are highly appreciated..
Answer for (i)
1. Explicit (unbound) cursor
EXECUTE is not a "clause", but a PL/pgSQL command to execute SQL strings. Cursors are not visible inside the command. You need to pass values to it.
Hence, you cannot use the special syntax WHERE CURRENT OFcursor. I use the system column ctid instead to determine the row without knowing the name of a unique column. Note that ctid is only guaranteed to be stable within the same transaction.
CREATE OR REPLACE FUNCTION f_curs1(_tbl text)
RETURNS void AS
$func$
DECLARE
_curs refcursor;
rec record;
BEGIN
OPEN _curs FOR EXECUTE 'SELECT * FROM ' || quote_ident(_tbl) FOR UPDATE;
LOOP
FETCH NEXT FROM _curs INTO rec;
EXIT WHEN rec IS NULL;
RAISE NOTICE '%', rec.tbl_id;
EXECUTE format('UPDATE %I SET tbl_id = tbl_id + 10 WHERE ctid = $1', _tbl)
USING rec.ctid;
END LOOP;
END
$func$ LANGUAGE plpgsql;
Why format() with %I?
There is also a variant of the FOR statement to loop through cursors, but it only works for bound cursors. We have to use an unbound cursor here.
2. Implicit cursor in FOR loop
There is normally no need for explicit cursors in plpgsql. Use the implicit cursor of a FOR loop instead:
CREATE OR REPLACE FUNCTION f_curs2(_tbl text)
RETURNS void AS
$func$
DECLARE
_ctid tid;
BEGIN
FOR _ctid IN EXECUTE 'SELECT ctid FROM ' || quote_ident(_tbl) FOR UPDATE
LOOP
EXECUTE format('UPDATE %I SET tbl_id = tbl_id + 100 WHERE ctid = $1', _tbl)
USING _ctid;
END LOOP;
END
$func$ LANGUAGE plpgsql;
3. Set based approach
Or better, yet (if possible!): Rethink your problem in terms of set-based operations and execute a single (dynamic) SQL command:
-- Set-base dynamic SQL
CREATE OR REPLACE FUNCTION f_nocurs(_tbl text)
RETURNS void AS
$func$
BEGIN
EXECUTE format('UPDATE %I SET tbl_id = tbl_id + 1000', _tbl);
-- add WHERE clause as needed
END
$func$ LANGUAGE plpgsql;
SQL Fiddle demonstrating all 3 variants.
Answer for (ii)
A schema-qualified table name like trace.myname actually consists of two identifiers. You have to
either pass and escape them separately,
or go with the more elegant approach of using a regclass type:
CREATE OR REPLACE FUNCTION f_nocurs(_tbl regclass)
RETURNS void AS
$func$
BEGIN
EXECUTE format('UPDATE %s SET tbl_id = tbl_id + 1000', _tbl);
END
$func$ LANGUAGE plpgsql;
I switched from %I to %s, because the regclass parameter is automatically properly escaped when (automatically) converted to text.
More details in this related answer:
Table name as a PostgreSQL function parameter