问题
I need to remove some attributes from a json type column.
The Table:
CREATE TABLE my_table( id VARCHAR(80), data json);
INSERT INTO my_table (id, data) VALUES (
'A',
'{"attrA":1,"attrB":true,"attrC":["a", "b", "c"]}'
);
Now, I need to remove attrB from column data.
Something like alter table my_table drop column data->'attrB'; would be nice. But a way with a temporary table would be enough, too.
回答1:
Update: for 9.5+, there are explicit operators you can use with jsonb (if you have a json typed column, you can use casts to apply a modification):
Deleting a key (or an index) from a JSON object (or, from an array) can be done with the - operator:
SELECT jsonb '{"a":1,"b":2}' - 'a', -- will yield jsonb '{"b":2}'
jsonb '["a",1,"b",2]' - 1 -- will yield jsonb '["a","b",2]'
Deleting, from deep in a JSON hierarchy can be done with the #- operator:
SELECT '{"a":[null,{"b":[3.14]}]}' #- '{a,1,b,0}'
-- will yield jsonb '{"a":[null,{"b":[]}]}'
For 9.4, you can use a modified version of the original answer (below), but instead of aggregating a JSON string, you can aggregate into a json object directly with json_object_agg().
Related: other JSON manipulations whithin PostgreSQL:
- How do I modify fields inside the new PostgreSQL JSON datatype?
Original answer (applies to PostgreSQL 9.3):
If you have at least PostgreSQL 9.3, you can split your object into pairs with json_each() and filter your unwanted fields, then build up the json again manually. Something like:
SELECT data::text::json AS before,
('{' || array_to_string(array_agg(to_json(l.key) || ':' || l.value), ',') || '}')::json AS after
FROM (VALUES ('{"attrA":1,"attrB":true,"attrC":["a","b","c"]}'::json)) AS v(data),
LATERAL (SELECT * FROM json_each(data) WHERE "key" <> 'attrB') AS l
GROUP BY data::text
With 9.2 (or lower) it is not possible.
Edit:
A more convenient form is to create a function, which can remove any number of attributes in a json field:
Edit 2: string_agg() is less expensive than array_to_string(array_agg())
CREATE OR REPLACE FUNCTION "json_object_delete_keys"("json" json, VARIADIC "keys_to_delete" TEXT[])
RETURNS json
LANGUAGE sql
IMMUTABLE
STRICT
AS $function$
SELECT COALESCE(
(SELECT ('{' || string_agg(to_json("key") || ':' || "value", ',') || '}')
FROM json_each("json")
WHERE "key" <> ALL ("keys_to_delete")),
'{}'
)::json
$function$;
With this function, all you need to do is to run the query below:
UPDATE my_table
SET data = json_object_delete_keys(data, 'attrB');
回答2:
This has gotten much easier with PostgreSQL 9.5 using the JSONB type. See JSONB operators documented here.
You can remove a top-level attribute with the "-" operator.
SELECT '{"a": {"key":"value"}, "b": 2, "c": true}'::jsonb - 'a'
// -> {"b": 2, "c": true}
You can use this within an update call to update an existing JSONB field.
UPDATE my_table SET data = data - 'attrB'
You can also provide the attribute name dynamically via parameter if used in a function.
CREATE OR REPLACE FUNCTION delete_mytable_data_key(
_id integer,
_key character varying)
RETURNS void AS
$BODY$
BEGIN
UPDATE my_table SET
data = data - _key
WHERE id = _id;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
The reverse operator is the "||", in order to concatenate two JSONB packets together. Note that the right-most use of the attribute will overwrite any previous ones.
SELECT '{"a": true, "c": true}'::jsonb || '{"a": false, "b": 2}'::jsonb
// -> {"a": false, "b": 2, "c": true}
回答3:
I couldn't get SELECT '{"a": "b"}'::jsonb - 'a'; to work in 9.5.2. However, SELECT '{"a": "b"}'::jsonb #- '{a}'; did work!
回答4:
While this is certainly easier in 9.5+ using the jsonb operators, the function that pozs wrote to remove multiple keys is still useful. For example, if the keys to be removed are stored in a table, you could use the function to remove them all. Here is an updated function, using jsonb and postgresql 9.5+:
CREATE FUNCTION remove_multiple_keys(IN object jsonb,
variadic keys_to_delete text[],
OUT jsonb)
IMMUTABLE
STRICT
LANGUAGE SQL
AS
$$
SELECT jsonb_object_agg(key, value)
FROM (SELECT key, value
FROM jsonb_each("object")
WHERE NOT (key = ANY("keys_to_delete"))
) each_subselect
$$
;
If the keys to be removed are stored in a table, (e.g. in the column "keys" of the table "table_with_keys") you could call this function like this:
SELECT remove_multiple_keys(my_json_object,
VARIADIC (SELECT array_agg(keys) FROM table_with_keys));
回答5:
It is an ugly hack but if attrB isn't your first key and it appears only once then you can do the following:
UPDATE my_table SET data = REPLACE(data::text, ',"attrB":' || (data->'attrB')::text, '')::json;
回答6:
One other convenient way of doing this is to use hstore extension. This way you can write some more convenient function to set/delete keys into a json object. I came up with following function to do the same:
CREATE OR REPLACE FUNCTION remove_key(json_in json, key_name text)
RETURNS json AS $$
DECLARE item json;
DECLARE fields hstore;
BEGIN
-- Initialize the hstore with desired key being set to NULL
fields := hstore(key_name,NULL);
-- Parse through Input Json and push each key into hstore
FOR item IN SELECT row_to_json(r.*) FROM json_each_text(json_in) AS r
LOOP
--RAISE NOTICE 'Parsing Item % %', item->>'key', item->>'value';
fields := (fields::hstore || hstore(item->>'key', item->>'value'));
END LOOP;
--RAISE NOTICE 'Result %', hstore_to_json(fields);
-- Remove the desired key from store
fields := fields-key_name;
RETURN hstore_to_json(fields);
END;
$$ LANGUAGE plpgsql
SECURITY DEFINER
STRICT;
A simple example of use is:
SELECT remove_key(('{"Name":"My Name", "Items" :[{ "Id" : 1, "Name" : "Name 1"}, { "Id" : 2, "Name 2" : "Item2 Name"}]}')::json, 'Name');
-- Result
"{"Items": "[{ \"Id\" : 1, \"Name\" : \"Name 1\"}, { \"Id\" : 2, \"Name 2\" : \"Item2 Name\"}]"}"
I have another function to do the set_key operation as well as following:
CREATE OR REPLACE FUNCTION set_key(json_in json, key_name text, key_value text)
RETURNS json AS $$
DECLARE item json;
DECLARE fields hstore;
BEGIN
-- Initialize the hstore with desired key value
fields := hstore(key_name,key_value);
-- Parse through Input Json and push each key into hstore
FOR item IN SELECT row_to_json(r.*) FROM json_each_text(json_in) AS r
LOOP
--RAISE NOTICE 'Parsing Item % %', item->>'key', item->>'value';
fields := (fields::hstore || hstore(item->>'key', item->>'value'));
END LOOP;
--RAISE NOTICE 'Result %', hstore_to_json(fields);
RETURN hstore_to_json(fields);
END;
$$ LANGUAGE plpgsql
SECURITY DEFINER
STRICT;
I have discussed this more in my blog here.
来源：https://stackoverflow.com/questions/23490965/postgresql-remove-attribute-from-json-column