I am using psql (PostgreSQL) 11.2 (Debian 11.2-1.pgdg90+1).
I am trying to write a logic in .PSQL file that needs to import some data into a table if this table is empty, else do something else.
I am struggling to find the correct syntax to make it work.
Would appreciate some help around this.
DO $$ BEGIN
SELECT count(*) from (SELECT 1 table_x LIMIT 1) as isTableEmpty
IF isTableEmpty > 0
THEN
INSERT INTO table_x
SELECT * FROM table_b;
ELSE
INSERT INTO table_y
SELECT * FROM table_b;
END IF;
END $$;
thanks!
Read plpgsql structure. Then you would know you need a DECLARE section to declare isTableEmpty and from here Select into that you need to select into the isTableEmpty variable. So:
...
DECLARE
isTableEmpty integer;
BEGIN
SELECT count(*) into isTableEmpty from (SELECT 1 table_x LIMIT 1);
...
Though I'm not sure what you are trying to accomplish with?:
SELECT count(*) from (SELECT 1 table_x LIMIT 1) as isTableEmpty
As that is always going to return 1.
You are using count just to determine that a row exists or not in the table. To do so you need to create a variable in the DO block, select into that variable, and reference that variable. This is all unnecessary; you can just use exists(...) instead of count(*) .... See demo;
do $$
begin
if not exists (select null from table_x) then
insert into table_x (...)
values (...);
else
insert into table_y (...)
values (...);
end if;
end ;
$$;
Related
I was attempting an INSERT INTO.... ( SELECT... ) (inserting a batch of rows from SELECT... subquery), onto the same table in my database. For the most part it was working, however, I did see a "Deadlock" exception logged every now and then. Does it make sense to do this or is there a way to avoid a deadlock scenario? On a high-level, my queries both resemble this structure:
CREATE OR REPLACE PROCEDURE myConcurrentProc() LANGUAGE plpgsql
AS $procedure$
DECLARE
BEGIN
LOOP
EXIT WHEN row_count = 0
WITH cte AS (SELECT *
FROM TableA tbla
WHERE EXISTS (SELECT 1 FROM TableB tblb WHERE tblb.id = tbla.id)
INSERT INTO concurrent_table (SELECT id FROM cte);
COMMIT;
UPDATE log_tbl
SET status = 'FINISHED',
WHERE job_name = 'tblA_and_B_job';
END LOOP;
END
$procedure$;
And the other script that runs in parallel and INSERTS... also to the same table is also basically:
CREATE OR REPLACE PROCEDURE myConcurrentProc() LANGUAGE plpgsql
AS $procedure$
DECLARE
BEGIN
LOOP
EXIT WHEN row_count = 0
WITH cte AS (SELECT *
FROM TableC c
WHERE EXISTS (SELECT 1 FROM TableD d WHERE d.id = tblc.id)
INSERT INTO concurrent_table (SELECT id FROM cte);
COMMIT;
UPDATE log_tbl
SET status = 'FINISHED',
WHERE job_name = 'tbl_C_and_D_job';
END LOOP;
END
$procedure$;
So you can see I'm querying two different tables in each script, however inserting into the same some_table. I also have the UPDATE... statement that writes to a log table so I suppose that could also cause issues. Is there any way to use BEGIN... END here and COMMIT to avoid any deadlock/concurrency issues or should I just create a 2nd table to hold the "tbl_C_and_D_job" data?
I have this select statement inside a trigger procedure:
SELECT 1 FROM some_table WHERE "user_id" = new."user_id"
AND created >= now()::date;
How can i store result in a variable and reuse it in IF statement like this:
IF NOT EXISTS (var_name) THEN ...;
procedure (for now i have select right in IF statement, but i want it separately)
CREATE OR REPLACE FUNCTION add_row() RETURNS TRIGGER AS $$
BEGIN
//need to check if row was created around today
IF NOT EXISTS (SELECT 1 FROM some_table WHERE "user_id" = new."user_id"
AND created >= now()::date) THEN
INSERT INTO another_table VALUES(1, 2, 3);
END IF;
END;
$$ LANGUAGE plpgsql;
To store the result of a query into a variable, you need to declare a variable. Then you can use select .. into .. to store the result. But I would use a boolean and an exists condition for this purpose.
CREATE OR REPLACE FUNCTION add_row()
RETURNS TRIGGER
AS $$
declare
l_row_exists boolean;
BEGIN
select exists (SELECT *
FROM some_table
WHERE user_id = new.user_id
AND created >= current_date)
into l_row_exists;
IF NOT l_row_exists THEN
INSERT INTO another_table (col1, col2, col3)
VALUES(1, 2, 3);
END IF;
END;
$$ LANGUAGE plpgsql;
However, you don't really need an IF statement to begin with. You can simplify this to a single INSERT statement:
INSERT INTO another_table (col1, col2, col3)
SELECT 1,2,3
WHERE NOT EXISTS (SELECT *
FROM some_table
WHERE user_id = new.user_id
AND created >= current_date);
Is there any way to replace the table name in a query with value stored in another table ? This is in postgres sql
Eg
Meta_table
col1 | col 2
Table 1 | val1
Table 2 | val2
My requirement
select *
from (select col1 from meta_table where col2 = val2)
Probably the most flexible and efficient way is dynamically creating a temporary view using function:
create or replace function f_prepare(tname text, vname text) returns text language plpgsql as $$
begin
execute format(
'create or replace temporary view %I as select * from %I',
vname, tname);
return vname;
end $$;
Then you can use the created view in usual way, for example;
select f_prepare('pg_class', 'v_class');
select * from v_class where relname = 'pg_database'; -- Index on the source table will be used here
and using your code:
select f_prepare((select col1 from meta_table where col2 = 'val2'), 'v');
select * from v;
And as any other temporary objects, created views will not conflict with other sessions and will be dropped on disconnect.
If you want to change the table name of a table, then you can just update the relname column in table pg_class.
But for this, you need admin access to the Postgresql.
The query goes like:-
update pg_class set relname='new_table_name' where relname='old_table_name';
So to do this in single line, You can do like this:
update pg_class set relname=(select col1 from meta_table where col2 = val2) where relname='old_table_name';
You can use Do statement with cursor:
Try This:
DO $$
DECLARE
_query text;
_cursor CONSTANT refcursor := '_cursor';
BEGIN
_query := 'SELECT * FROM '|| (select col1 from meta_table where col2 = 'val1');
OPEN _cursor FOR EXECUTE _query;
END
$$;
FETCH ALL FROM _cursor;
I want to select a column from a table, with the column name being the result of a query like the following:
-- This query returns a single value
with x as (
select a from table1 where <condition>
)
-- my_function() yields a table
select x from my_function()
How do I do that?
Thank you very much.
You could write it in SQL with a temporary function:
CREATE FUNCTION pg_temp.tablefunc()
RETURNS SETOF my_function_result_type
LANGUAGE plpgsql AS
$$DECLARE
v_colname text;
BEGIN
SELECT a INTO v_colname
FROM table1
LIMIT 1;
RETURN QUERY EXECUTE
format(E'SELECT %I\n'
'FROM my_function()',
v_colname);
END;$$;
SELECT * FROM pg_temp.tablefunc();
I have multiple tables with each two rows of interest: connection_node_start_id and connection_node_end_id. My goal is to get a collection of all those IDs, either as a flat ARRAY or as a new TABLE consisting of one row.
Example output ARRAY:
result = {1,4,7,9,2,5}
Example output TABLE:
IDS
-------
1
4
7
9
2
5
My fist attempt is somewhat clumsy and does not work properly as the SELECT statement just returns one row. It seems there must be a simple way to do this, can someone point me into the right direction?
CREATE OR REPLACE FUNCTION get_connection_nodes(anyarray)
RETURNS anyarray AS
$$
DECLARE
table_name varchar;
result integer[];
sel integer[];
BEGIN
FOREACH table_name IN ARRAY $1
LOOP
RAISE NOTICE 'table_name(%)',table_name;
EXECUTE 'SELECT ARRAY[connection_node_end_id,
connection_node_start_id] FROM ' || table_name INTO sel;
RAISE NOTICE 'sel(%)',sel;
result := array_cat(result, sel);
END LOOP;
RETURN result;
END
$$
LANGUAGE 'plpgsql';
Test table:
connection_node_start_id | connection_node_end_id
--------------------------------------------------
1 | 4
7 | 9
Call:
SELECT get_connection_nodes(ARRAY['test_table']);
Result:
{1,4} -- only 1st row, rest is missing
For Postgres 9.3+
CREATE OR REPLACE FUNCTION get_connection_nodes(text[])
RETURNS TABLE (ids int) AS
$func$
DECLARE
_tbl text;
BEGIN
FOREACH _tbl IN ARRAY $1
LOOP
RETURN QUERY EXECUTE format('
SELECT t.id
FROM %I, LATERAL (VALUES (connection_node_start_id)
, (connection_node_end_id)) t(id)'
, _tbl);
END LOOP;
END
$func$ LANGUAGE plpgsql;
Related answer on dba.SE:
SELECT DISTINCT on multiple columns
Or drop the loop and concatenate a single query. Probably fastest:
CREATE OR REPLACE FUNCTION get_connection_nodes2(text[])
RETURNS TABLE (ids int) AS
$func$
BEGIN
RETURN QUERY EXECUTE (
SELECT string_agg(format(
'SELECT t.id FROM %I, LATERAL (VALUES (connection_node_start_id)
, (connection_node_end_id)) t(id)'
, tbl), ' UNION ALL ')
FROM unnest($1) tbl
);
END
$func$ LANGUAGE plpgsql;
Related:
Loop through like tables in a schema
LATERAL was introduced with Postgres 9.3.
For older Postgres
You can use the set-returning function unnest() in the SELECT list, too:
CREATE OR REPLACE FUNCTION get_connection_nodes2(text[])
RETURNS TABLE (ids int) AS
$func$
BEGIN
RETURN QUERY EXECUTE (
SELECT string_agg(
'SELECT unnest(ARRAY[connection_node_start_id
, connection_node_end_id]) FROM ' || tbl
, ' UNION ALL '
)
FROM (SELECT quote_ident(tbl) AS tbl FROM unnest($1) tbl) t
);
END
$func$ LANGUAGE plpgsql;
Should work with pg 8.4+ (or maybe even older). Works with current Postgres (9.4) as well, but LATERAL is much cleaner.
Or make it very simple:
CREATE OR REPLACE FUNCTION get_connection_nodes3(text[])
RETURNS TABLE (ids int) AS
$func$
BEGIN
RETURN QUERY EXECUTE (
SELECT string_agg(format(
'SELECT connection_node_start_id FROM %1$I
UNION ALL
SELECT connection_node_end_id FROM %1$I'
, tbl), ' UNION ALL ')
FROM unnest($1) tbl
);
END
$func$ LANGUAGE plpgsql;
format() was introduced with pg 9.1.
Might be a bit slower with big tables because each table is scanned once for every column (so 2 times here). Sort order in the result is different, too - but that does not seem to matter for you.
Be sure to sanitize escape identifiers to defend against SQL injection and other illegal syntax. Details:
Table name as a PostgreSQL function parameter
The EXECUTE ... INTO statement can only return data from a single row:
If multiple rows are returned, only the first will be assigned to the INTO variable.
In order to concatenate values from all rows you have to aggregate them first by column and then append the arrays:
EXECUTE 'SELECT array_agg(connection_node_end_id) ||
array_agg(connection_node_start_id) FROM ' || table_name INTO sel;
You're probably looking for something like this:
CREATE OR REPLACE FUNCTION d (tblname TEXT [])
RETURNS TABLE (c INTEGER) AS $$
DECLARE sql TEXT;
BEGIN
WITH x
AS (SELECT unnest(tblname) AS tbl),
y AS (
SELECT FORMAT('
SELECT connection_node_end_id
FROM %s
UNION ALL
SELECT connection_node_start_id
FROM %s
', tbl, tbl) AS s
FROM x)
SELECT string_agg(s, ' UNION ALL ')
INTO sql
FROM y;
RETURN QUERY EXECUTE sql;
END;$$
LANGUAGE plpgsql;
CREATE TABLE a (connection_node_end_id INTEGER, connection_node_start_id INTEGER);
INSERT INTO A VALUES (1,2);
CREATE TABLE b (connection_node_end_id INTEGER, connection_node_start_id INTEGER);
INSERT INTO B VALUES (100, 101);
SELECT * from d(array['a','b']);
c
-----
1
2
100
101
(4 rows)