This can be done, very efficiently, too. Not in a single statement, though, since SQL demands to know the return type at call time. So you need two steps. The solution involves a number of advanced techniques ...
Assuming the same table as @Denver in his answer:
CREATE TABLE hstore_test (
id serial PRIMARY KEY
, hstore_col hstore
);
Solution 1: Simple SELECT
After I wrote the crosstab solution below it struck me that a simple "brute force" solution is probably faster. Basically, the query @Denver already posted, built dynamically:
Step 1a: Generate query
SELECT format(
'SELECT id, h->%s
FROM (SELECT id, hstore_col AS h FROM hstore_test) t;'
, string_agg(quote_literal(key) || ' AS ' || quote_ident(key), ', h->')
) AS sql
FROM (
SELECT DISTINCT key
FROM hstore_test, skeys(hstore_col) key
ORDER BY 1
) sub;
The subquery (SELECT id, hstore_col AS h FROM hstore_test)
is just to get in the column alias h
for your hstore
column.
Step 1b: Execute query
This generates a query of the form:
SELECT id, h->'key1' AS key1, h->'key2' AS key2, h->'key3' AS key3
FROM (SELECT id, hstore_col AS h FROM hstore_test) t;
Result:
id | key1 | key2 | key3
----+-------+-------+-------
1 | val11 | val12 | val13
2 | val21 | val22 |
3 | | | -- for a row where hstore_col IS NULL
Solution 2: crosstab()
For lots of keys this may perform better. Probably not. You'll have to test. Result is the same as for solution 1.
You need the additional extension tablefunc
which provides the crosstab()
function. Read this first if you are not familiar:
Step 2a: Generate query
SELECT format(
$s$SELECT * FROM crosstab(
$$SELECT h.id, kv.*
FROM hstore_test h, each(hstore_col) kv
ORDER BY 1, 2$$
, $$SELECT unnest(%L::text[])$$
) AS t(id int, %s text);
$s$
, array_agg(key) -- escapes strings automatically
, string_agg(quote_ident(key), ' text, ') -- needs escaping!
) AS sql
FROM (
SELECT DISTINCT key
FROM hstore_test, skeys(hstore_col) key
ORDER BY 1
) sub;
Note the nested levels of dollar-quoting.
I use this explicit form in the main query instead of the short CROSS JOIN
in the auxiliary query to preserve rows with empty or NULL hstore
values:
LEFT JOIN LATERAL each(hstore_col) kv ON TRUE
Related:
Step 2b: Execute query
This generates a query of the form:
SELECT * FROM crosstab(
$$SELECT h.id, kv.*
FROM hstore_test h
LEFT JOIN LATERAL each(hstore_col) kv ON TRUE
ORDER BY 1, 2$$
, $$SELECT unnest('{key1,key2,key3}'::text[])$$
) AS t(id int, key1 text, key2 text, key3 text);
You may want to inspect it for plausibility before running the first time. This should deliver optimized performance.
Notes