I've been using Postgres to store JSON objects as strings, and now I want to utilize PG's built-in json and jsonb types to store the objects more efficiently.
Basically, I want to parse the stringified JSON and put it in a json column from within PG, without having to resort to reading all the values into Python and parsing them there.
Ideally, my migration should look like this:
UPDATE table_name SET json_column=parse_json(string_column);
I looked at Postgres's JSON functions, and there doesn't seem to be a method of doing this, even though it seems pretty trivial. For the record, my JSON objects are just one-dimensional arrays of strings.
Is there any way to do this?
There is no need for a parse_json column, just change the type of the column:
ALTER TABLE table_name
ALTER COLUMN json_column TYPE json USING json_column::json;
Note that if you plan on doing a lot of JSON operations on these values (i.e. extracting elements from objects, modifying objects etc) it's better to use jsonb. json should only be used for storing JSON data. Also, as Laurenz Albe points out, if you don't need to do any JSON operations on these values and you are not interested in the validation that postgresql can do on them (e.g. because you trust that the source always provides valid JSON), then using text is a perfectly valid option (or bytea).
Related
Is it possible to create a json schema for an array with undefined length (besides it always being an even number of elements) that captures a map stored as an array?
i.e. as described in title [key1, val1, key2, val2.....]
it seems that the only option for an array of an undetermined length is to have a single item "type" (though that type could conceptually be a oneOf type). However, that wouldn't enforce ordering of key/val schema restrictions. While it would validate valid uses, it would also validate invalid uses.
if I knew how long the array would be, I could just enforce it by specifying the types for all keys and values in their respective positions, but that's not the case here.
Yes, it would be nice if the api worked off a map/object instead of an array in this location, but this is an old api that I'm trying to create a json schema for, so it probably can't be changed.
A project I'm working on involves storing a string of data in a table column. The table will have other columns relevant to the records. We decided to store the string data column using JSON.
From the table, a view will parse the JSON column into separate columns. The view will also have columns derived from the other main table columns. The data from the view is then used to populate parts of a document through SSRS.
When loading data into the main table, I need to utilize separate tables for deriving the other column values and the JSON column. I decided to use common table expressions for this. At the end of the query, I bring together the derived columns from the different common table expressions, including the JSON column, and insert them into the main table.
I had it almost done until I realized that when I use FOR JSON to create the JSON column, it escapes special characters. I did some research and have been trying to use the JSON_QUERY function to get around this but it's not working. Here is a simplification of the problem:
WITH Table1
(
First_Name_JSON
)
As
(
SELECT 'Tim/' As First_Name
FOR JSON PATH
)
SELECT JSON_QUERY(Table1.First_Name_JSON) as first_name
FROM Table1
FOR JSON PATH
Here is the output:
[{"first_name":[{"First_Name":"Tim\/"}]}]
Why is it still escaping? The documentation shows that passing a column that was created by a FOR JSON should make the JSON_QUERY function return it without escaped characters.
I know that this works:
SELECT JSON_QUERY('{"Firt_Name": "Tim/"}') as first_name
FOR JSON PATH
Output:
[{"first_name":{"Firt_Name": "Tim/"}}]
However, I need to be able to pass a column that's holding JSON data already because it's pretty long logic with many columns. Using FOR JSON is ideal for making changes versus hard coding the JSON format around each column.
I must be missing something. Thanks for any help.
It's quite simple:
{"Firt_Name": "Tim/"} is valid JSON, so JSON_QUERY can return it as is. Tim/ is not valid so needs escaping first.
Quote from the docs:
Using JSON_QUERY with FOR JSON
JSON_QUERY returns a valid JSON fragment. As a result, FOR JSON doesn't escape special characters in the JSON_QUERY return value.
If you're returning results with FOR JSON, and you're including data that's already in JSON format (in a column or as the result of an expression), wrap the JSON data with JSON_QUERY without the path parameter.
Given your use case, is it not possible to pass through the JSON to where you need it and un-escape it there? OPENJSON and JSON_VALUE are capable of this.
I need to store an array of strings as a table column in Redshift and then check whether this array contains some string.
Since Redshift doesn't support array types, I started looking for ways around it. The only thing I came up is to encode this array as a pipe-separated string, previously escaping all the pipes within the elements of the array. Looking up of the element will be done using regexps.
While this solution seems to be viable, it requires some pre- and post-processing. Maybe you can recommend some alternatives?
I have totally rewritten my question because of inaccurate description of the problem!
We have to store a lot of different informations about a specific region. For this we need a flexible data structure which does not limit the possibilities for the user.
So we've create a key-value table for this additional data which is described through a meta table which contains the datatype of the value.
We already use this information for queries over our rest api. We then automatically wrap the requested field with into a cast.
SQL Fiddle
We return this data together with information form other tables as a JSON object. We convert the corresponding rows from the data-table with array_agg and json_object into a JSON object:
...
CASE
WHEN count(prop.name) = 0 THEN '{}'::json
ELSE json_object(array_agg(prop.name), array_agg(prop.value))
END AS data
...
This works very well. Now the problem we have is if we store data like a floating point number into this field, we then get returned a string representation of this number:
e.g. 5.231 returns as "5.231"
Now we would like to CAST this number during our select statement into the right data-format so the JSON result would be correctly formatted. We have all the information we need so we tried following:
SELECT
json_object(array_agg(data.name),
-- here I cast the value into the right datatype!
-- results in an error
array_agg(CAST(value AS datatype))) AS data
FROM data
JOIN (
SELECT name, datatype
FROM meta)
AS info
ON info.name = data.name
The error message is following:
ERROR: type "datatype" does not exist
LINE 3: array_agg(CAST(value AS datatype))) AS data
^
Query failed
PostgreSQL said: type "datatype" does not exist
So is it possible to dynamically cast the text of the data_type column to a postgresql type to return a well-formatted JSON object?
First, that's a terrible abuse of SQL, and ought to be avoided in practically all scenarios. If you have a scenario where this is legitimate, you probably already know your RDBMS so intimately, that you're writing custom indexing plugins, and wouldn't even think of asking this question...
If you tell us what you're actually trying to do, there's about a 99.9% chance we can tell you a better way to do it.
Now with that disclaimer aside:
This is not possible, without using dynamic SQL. With a sufficiently recent version of PostgreSQL, you can accomplish this with the use of 'EXECUTE IMMEDIATE', which you can read about in the manual. It basically boils down to using EXEC.
Note, however, that even using this method, the result for every row fetched in the same query must have the same data type. In other words, you can't expect that row 1 will have a data type of VARCHAR, and row 2 will have INT. That is completely impossible.
The problem you have is, that json_object does create an object out of a string array for the keys and another string array for the values. So if you feed your JSON objects into this method, it will always return an error.
So the first problem is, that you have to use a JSON or JSONB column for the values. Or you can convert the values from string to json with to_json().
Now the second problem is that you need to use another method to create your json object because you want to feed it with a string array for the keys and a json-object array for the values. For this there is a method called json_object_agg.
Then your output should be like the one you expected! Here the full query:
SELECT
json_object_agg(data.name, to_json(data.value)) AS data
FROM data
I am getting a JSON response and would like to add it to an SQLite db. The results come back as an array and in each array there will be about 30 keys with values. What would be the most efficient approach to adding all those to my table? The keys coming from the JSON would be the columns in the sqlite db.
Would it be best to do a for loop on each array item then another for loop in side to get the values and add it to a string then add them to the database that way? Or is there a better approach using FMDB to add a JSON response directly to the database if the json keys match the database table columns?
If you believe your JSON response will not change and your data model will not change (or rarely change), then I'd just loop through the arrays and write the slightly long...
[db executeUpdate:#"INSERT INTO response (key1,key2,..key30) VALUES (?,?,?...etc);", json_reponse.value1,json_response.value2,...,json_response.value30, nil];
However, if this model would change, be extended, etc... then I'd probably just use Core Data.
The biggest factor though is what are you doing with the data after it is stored? Creating objects, display a report, converting the objects back to JSON (then just store the raw JSON in a text field)?
I ended up doing a loop and getting the json keys and using those as the columns.