In my table mytable I have a json field called data and I inserted json with a lot of keys & values.
I know that it's possible to select individual fields like so:
SELECT data->'mykey' as mykey from mytable
But how can I get an overview of all of the json keys on a certain depth? I would have expected something like
SELECT data->* from mytable
but that doesn't work. Is there something similar?
You can use the json_object_keys() function to get all the top-level keys of a json value:
SELECT keys.*
FROM mytable, json_object_keys(mytable.data) AS keys (mykey);
If you want to search at a deeper level, then first extract that deeper level from the json value using the #> operator:
SELECT keys.*
FROM mytable, json_object_keys(mytable.data #> '{level1, level2}') AS keys (mykey);
Note that the function returns a set of text, so you should invoke the function as a row source.
If you are using the jsonb data type, then use the jsonb_object_keys() function.
Related
In a Postgres database I have a column of type jsonb which contains a generic object whose keys I cannot predict:
{"some_key":"value", "another_unpredictable_key":"another value}
Is it possible to search for a particular value in all of the fields without knowing the keys? Sth. like
select * from ... where column_whatever->>'*' = '...'
You need to turn the json value into multiple rows (of key/value pairs) and search in the result of that:
select *
from some_table t
where exists (select *
from jsonb_each_text(t.jsonb_column) as x(ky,val)
where x.val = 'some value');
jsonb_each_text() returns one row for each top-level key/value pair. This does not handle nested keys.
I have a jsonb field in a PostgreSQL table which was supposed to contain a dictionary like data aka {} but few of its entries got an array due to source data issues.
I want to weed out those entries. One of the ways is to perform following query -
select json_field from data_table where cast(json_field as text) like '[%]'
But this requires converting each jsonb field into text. With data_table having order of 200 million entries, this looks like bit of an overkill.
I investigated pg_typeof but it returns jsonb which doesn't help differentiate between a dictionary and an array.
Is there a more efficient way to achieve the above?
How about using the json_typeof function?
select json_field from data_table where json_typeof(json_field) = 'array'
I have a table users with a jsonb field called data. I have to retrieve all the users that have a value in that data column matching a given string. For example:
user1 = data: {"property_a": "a1", "property_b": "b1"}
user2 = data: {"property_a": "a2", "property_b": "b2"}
I want to retrieve any user that has a value data matching 'b2', in this case that will be 'user2'.
Any idea how to do this in an elegant way? I can retrieve all keys from data of all users and create a query manually but that will be neither fast nor elegant.
In addition, I have to retrieve the key and value matched, but first things first.
There is no easy way. Per documentation:
GIN indexes can be used to efficiently search for keys or key/value
pairs occurring within a large number of jsonb documents (datums)
Bold emphasis mine. There is no index over all values. (Those can have non-compatible data types!) If you do not know the name(s) of all key(s) you have to inspect all JSON values in every row.
If there are just two keys like you demonstrate (or just a few well-kown keys), it's still easy enough:
SELECT *
FROM users
WHERE data->>'property_a' = 'b2' OR
data->>'property_b' = 'b2';
Can be supported with a simple expression index:
CREATE INDEX foo_idx ON users ((data->>'property_a'), (data->>'property_b'))
Or with a GIN index:
SELECT *
FROM users
WHERE data #> '{"property_a": "b2"}' OR
data #> '{"property_b": "b2"}'
CREATE INDEX bar_idx ON users USING gin (data jsonb_path_ops);
If you don't know all key names, things get more complicated ...
You could use jsonb_each() or jsonb_each_text() to unnest all values into a set and then check with an ANY construct:
SELECT *
FROM users
WHERE jsonb '"b2"' = ANY (SELECT (jsonb_each(data)).value);
Or
...
WHERE 'b2' = ANY (SELECT (jsonb_each_text(data)).value);
db<>fiddle here
But there is no index support for the last one. You could instead extract all values into and array and create an expression index on that, and match that expression in queries with array operators ...
Related:
How do I query using fields inside the new PostgreSQL JSON datatype?
Index for finding an element in a JSON array
Can PostgreSQL index array columns?
Try this query.
SELECT * FROM users
WHERE data::text LIKE '%b2%'
Of course it won't work if your key will contain such string too.
One of the great things about postgres is that it allows indexing into a json object.
I have a column of data formatted a little bit like this:
{"Items":
[
{"RetailPrice":6.1,"EffectivePrice":0,"Multiplier":1,"ItemId":"53636"},
{"RetailPrice":0.47,"EffectivePrice":0,"Multiplier":1,"ItemId":"53404"}
]
}
What I'd like to do is find the average RetailPrice of each row with these data.
Something like
select avg(json_extract_path_text(item_json, 'RetailPrice'))
but really I need to do this for each item in the items json object. So for this example, the value in the queried cell would be 3.285
How can I do this?
Could work like this:
WITH cte(tbl_id, json_items) AS (
SELECT 1
, '{"Items": [
{"RetailPrice":6.1,"EffectivePrice":0,"Multiplier":1,"ItemId":"53636"}
,{"RetailPrice":0.47,"EffectivePrice":0,"Multiplier":1,"ItemId":"53404"}]}'::json
)
SELECT tbl_id, round(avg((elem->>'RetailPrice')::numeric), 3) AS avg_retail_price
FROM cte c
, json_array_elements(c.json_items->'Items') elem
GROUP BY 1;
The CTE just substitutes for a table like:
CREATE TABLE tbl (
tbl_id serial PRIMARY KEY
, json_items json
);
json_array_elements() (Postgres 9.3+) to unnest the json array is instrumental.
I am using an implicit JOIN LATERAL here. Much like in this related example:
Query for element of array in JSON column
For an index to support this kind of query consider this related answer:
Index for finding an element in a JSON array
For details on how to best store EAV data:
Is there a name for this database structure?
I have a table that has a JSON list as one of its values. The column name is list_order and the value would be something like: [1,2,3].
I am having trouble doing a WHERE comparison to select by list_order. In pure SQL, it would be: SELECT * FROM table_name list_order=[1,2,3];
The closest example I found was this: How do I query using fields inside the new PostgreSQL JSON datatype?. However, this grabs the value of a key in the JSON where the JSON is a dictionary and not a list. I've tried modifying it to suit my need but it did not work.
Any suggestions? Is that even possible? Why is not documented? Thanks!
I found the answer. I need to compare it as text:
"SELECT * FROM table WHERE list_order::text='[1,2,3]';