Detect JSON columns

We should be able to automatically detect if a column contains JSON data and avoid escaping it in the output. Right now if I have the following table:

create table s003b.todos
(
	id int not null constraint pk__s003b_todos primary key default (next value for s003b.globalId),
	category_id varchar(3) collate Latin1_General_BIN2 not null constraint fk__s003b_todos__s003b_categories references s003b.categories(id),
	title nvarchar(100) not null,
	[description] nvarchar(1000) null,
	tags nvarchar(max) null check ( isjson(tags)= 1 ),
	completed bit not null default(0)
)

and I store JSON data in the “tags” column, I get the following output:

{ data": {
		"todos": {
			"items": [
				{
					"id": 10000,
					"title": "item-001",
					"completed": false,
					"tags": "[{\"tag\":\"red\"}]",
					"category": {
						"id": "f",
						"category": "Family"
					}
				}

where tags contains encoded JSON…even if the content is valid JSON itself. Right now Azure SQL DB doesn’t have a native JSON data type, but we can check if a column contains JSON data by checking the check constraint that should have been created to allow only JSON data to be inserted (see table definition above). This query can return which columns should be treated as JSON:

select 
	s.[name] as [schema_name],
	t.[name] as [table_name],
	c.[name] as [columne_name],
	ck.[definition],
	case when (ck.[definition] like '%isjson(/[' + trim(c.[name]) + '/])=(1)%' escape '/') then 1 else 0 end as [isjson]
from 
	sys.check_constraints ck
inner join
	sys.tables t on ck.[parent_object_id] = t.[object_id]
inner join
	sys.columns c on t.[object_id] = c.[object_id] and ck.parent_column_id = c.[column_id]
inner join
	sys.schemas s on t.[schema_id] = s.[schema_id]

[aaronpowell]

Because curiosity meant that I needed to dig into this more I decided to look at what we need to do to identify JSON columns.

PostgreSQL (and I expect MySQL but didn’t test) should be reasonably easy to detect as they have a column type of json that is returned when using the conn.GetSchema("Columns") query:

Azure SQL is a little harder as it isn’t a data type but it’s a constraint on the table that we can query for and then find the column that the constraint is applicable to. Here’s a SQL query that we can execute:

select schema_name(t.schema_id) + '.' + t.[name]  as [table],
    col.[name] as column_name,
    con.[definition]
from sys.check_constraints as con
    left outer join sys.objects as t
        on con.parent_object_id = t.object_id
    left outer join sys.all_columns as col
        on con.parent_column_id = col.column_id
        and con.parent_object_id = col.object_id
where lower(con.[definition]) like '%isjson%'
and con.is_disabled = 0

With this we could add another property to ColumnDefinition of IsJson (or similar) and use that as part of the codegen for the GraphQL engine.

[Aniruddh25]

Not enough time to do this with competing deadlines in Jan203. I agree let’s do 1 since we need to support Synapse anyway and its unfortunate that ADO.NET doesn’t have that support yet.