Dynamic SELECT INTO in PL/pgSQL function - postgresql

Question

How can I write a dynamic SELECT INTO query inside a PL/pgSQL function in Postgres?
Say I have a variable called tb_name which is filled in a FOR loop from information_schema.tables. Now I have a variable called tc which will be taking the row count for each table. I want something like the following:
FOR tb_name in select table_name from information_schema.tables where table_schema='some_schema' and table_name like '%1%'
LOOP
EXECUTE FORMAT('select count(*) into' || tc 'from' || tb_name);
END LOOP
What should be the data type of tb_name and tc in this case?

Answer 1

CREATE OR REPLACE FUNCTION myfunc(_tbl_pattern text, _schema text = 'public')
RETURNS void AS -- or whatever you want to return
$func$
DECLARE
_tb_name information_schema.tables.table_name%TYPE; -- currently varchar
_tc bigint; -- count() returns bigint
BEGIN
FOR _tb_name IN
SELECT table_name
FROM information_schema.tables
WHERE table_schema = _schema
AND table_name ~ _tbl_pattern -- see below!
LOOP
EXECUTE format('SELECT count(*) FROM %I.%I', _schema, _tb_name)
INTO _tc;
-- do something with _tc
END LOOP;
END
$func$ LANGUAGE plpgsql;
Notes
I prepended all parameters and variables with an underscore (_) to avoid naming collisions with table columns. Just a useful convention.
_tc should be bigint, since that's what the aggregate function count() returns.
The data type of _tb_name is derived from its parent column dynamically: information_schema.tables.table_name%TYPE. See the chapter Copying Types in the manual.
Are you sure you only want tables listed in information_schema.tables? Makes sense, but be aware of implications. See:
How to check if a table exists in a given schema
a_horse already pointed to the manual and Andy provided a code example. This is how you assign a single row or value returned from a dynamic query with EXECUTE to a (row) variable. A single column (like count in the example) is decomposed from the row type automatically, so we can assign to the scalar variable tc directly - in the same way we would assign a whole row to a record or row variable. Related:
How to get the value of a dynamically generated field name in PL/pgSQL
Schema-qualify the table name in the dynamic query. There may be other tables of the same name in the current search_path, which would result in completely wrong (and very confusing!) results without schema-qualification. Sneaky bug! Or this schema is not in the search_path at all, which would make the function raise an exception immediately.
How does the search_path influence identifier resolution and the "current schema"
Always quote identifiers properly to defend against SQL injection and random errors. Schema and table have to be quoted separately! See:
Table name as a PostgreSQL function parameter
Truncating all tables in a Postgres database
I use the regular expression operator ~ in table_name ~ _tbl_pattern instead of table_name LIKE ('%' || _tbl_pattern || '%'), that's simpler. Be wary of special characters in the pattern parameter either way! See:
PostgreSQL Reverse LIKE
Escape function for regular expression or LIKE patterns
Pattern matching with LIKE, SIMILAR TO or regular expressions in PostgreSQL
I set a default for the schema name in the function call: _schema text = 'public'. Just for convenience, you may or may not want that. See:
Assigning default value for type
Addressing your comment: to pass values, use the USING clause like:
EXECUTE format('SELECT count(*) FROM %I.%I
WHERE some_column = $1', _schema, _tb_name,column_name)
USING user_def_variable;
Related:
INSERT with dynamic table name in trigger function

Answer 2

It looks like you want the %I placeholder for FORMAT so that it treats your variable as an identifier. Also, the INTO clause should go outside the prepared statement.
FOR tb_name in select table_name from information_schema.tables where table_schema='some_schema' and table_name like '%1%'
LOOP
EXECUTE FORMAT('select count(*) from %I', tb_name) INTO tc;
END LOOP