A column-safe `INSERT INTO t1 SELECT * FROM ...`? - sql

Question

Is there some way to do a INSERT INTO t1 SELECT * FROM... such that it fails if the column names do not coincide?
I'm using Postgresql 9.x The columns names are not known in advance.
Motivation: I'm doing a periodic refresh of materialized views by the (quite standard) PL/pgSQL procedure:
CREATE OR REPLACE FUNCTION matview_refresh(name) RETURNS void AS
$BODY$
DECLARE
matview ALIAS FOR $1;
entry matviews%ROWTYPE;
BEGIN
SELECT * INTO entry FROM matviews WHERE mv_name = matview;
IF NOT FOUND THEN
RAISE EXCEPTION 'Materialized view % does not exist.', matview;
END IF;
EXECUTE 'TRUNCATE TABLE ' || matview;
EXECUTE 'INSERT INTO ' || matview || ' SELECT * FROM ' || entry.v_name;
UPDATE matviews SET last_refresh=CURRENT_TIMESTAMP WHERE mv_name=matview;
RETURN;
END
I preferred a TRUNCATE followed by a SELECT * INTO instead of a DROP/CREATE because it seemed more light and concurrent-friendly. It would fail if someone adds/remove columns from the view (then I would do the DROP/CREATE) but, it doesn't matter, in that case the refresh would not complete and we would catch the problem soon. What does matter is what happened today: someone changed the order of two columns of the view (of the same type), and the refresh inserted bogus data.

Answer 1

Build this into your plpgsql function to verify that view and table share the same column names in the same sequence exactly:
IF EXISTS (
SELECT 1
FROM (
SELECT *
FROM pg_attribute
WHERE attrelid = matview::regclass
AND attisdropped = FALSE
AND attnum > 0
) t
FULL OUTER JOIN (
SELECT *
FROM pg_attribute
WHERE attrelid = entry.v_name::regclass
AND attisdropped = FALSE
AND attnum > 0
) v USING (attnum, attname) -- atttypid to check for type, too
WHERE t.attname IS NULL
OR v.attname IS NULL
) THEN
RAISE EXCEPTION 'Mismatch between table and view!';
END IF;
The FULL OUTER JOIN adds a row with NULL values for any mismatch between the list of column names. So, if EXISTS finds a row, something is off.
And the cast to ::regclass would raise an exception right away if either table or view do not exist (or is out of scope - not in the search_path and not schema-qualified).
If you also want to check data types of the columns, just add atttypid to the USING clause.
As an aside: Querying pg_catalog tables is regularly faster by an order of magnitude than querying the bloated views int information_schema - information_schema is only good for SQL standard compliance and portability of code. Since you are writing 100 % Postgres-specific code, neither is relevant here.

Answer 2

You can query information_schema.columns to get columns in the right order:
SELECT INTO cols array_to_string(array_agg(column_name::text), ',')
FROM (
SELECT column_name
FROM information_schema.columns
WHERE table_name = 'matview'
ORDER BY ordinal_position
) AS x;
EXECUTE 'INSERT INTO ' || matview || ' SELECT ' || cols || ' FROM ' || entry.v_name;
You can get column list directly from pg_attribute -- just replace inner SELECT from information_schema.columns by:
SELECT attname AS column_name
FROM pg_attribute
WHERE attrelid = 'matview'::regclass AND attisdropped = false
ORDER BY attnum;