2020 – Part 2 – Find JSON Columns In Describe Function

The first posts in this series set up the data, the Polymorphic Table Function (PTF) and the Dataguide Search Index. This posts starts putting it all together by finding the JSON columns passed into the PTF.

Its done in the DESCRIBE function with this code:

-- for every column passed in
FOR counter IN 1..p_tbl.column.count LOOP
  -- is this a JSON Data Guide column?
    SELECT 1
      INTO v_dummy
      FROM user_json_dataguides
     WHERE column_name = 
TRIM(BOTH '"' FROM p_tbl.column(counter).description.name);
         p_tbl.column(counter).description.name || 
                      'is a JSON Dataguide Field');
  END; -- JSON column
END LOOP; -- every column passed in

Its pretty straightforward:

  1. Get a column name from the passed in query
  2. Check if it is in the USER_JSON_DATAGUIDES view
  3. If it is then output a string

I’ll expand the functionality in future posts but right now I want to make an important point – I assume the query passed in belongs to the current schema by querying the USER view. Will that always be valid? I dont know yet – but I will cover it in future posts.

Another point I want to make is that column names have quotes around them as shown in the output from running the PTF.

"JSON_DATA"is a JSON Dataguide Field

In the next post we’ll start extracting the KEY’s from the JSON.

Thanks for reading!

2020 – Part 1 – Dataguide Views

The next thing we need for Project 2020 is a simple and fast way to determine what keys are contained in our JSON data. My favourite way to get this is an Oracle Search Indexes because they are:


The SQL to create them is very straightforward and accessing the JSON keys simple via views in the database.


The views are in the database, right next to the JSON data itself so access is very fast.

Let’s see how they work…

First up – here is the syntax for creating a search index with the DATAGUIDE ON syntax telling Oracle to maintain the views for it.

CREATE SEARCH INDEX search_stuff ON json_stuff ( json_data )

The first view of the JSON data is provided by the JSON_DATAGUIDES.

SELECT dataguide
  FROM user_json_dataguides
 WHERE table_name = 'JSON_STUFF'
   AND column_name = 'JSON_DATA';

Here is the output.

"o:path" : "$.KEY1",
"type" : "string",
"o:length" : 8,
"o:preferred_column_name" : "JSON_DATA$KEY1"
"o:path" : "$.KEY2",
"type" : "string",
"o:length" : 8,
"o:preferred_column_name" : "JSON_DATA$KEY2"
"o:path" : "$.KEY3",
"type" : "string",
"o:length" : 8,
"o:preferred_column_name" : "JSON_DATA$KEY3"

That’s JSON about the JSON! Being a relational database person I much prefer the second view – JSON_DATAGUIDE_FIELDS shown below.

SELECT path,
  FROM user_json_dataguide_fields
 WHERE table_name = 'JSON_STUFF'
   AND column_name = 'JSON_DATA';

Here is output.

PATH       TYPE       LENGTH
---------- ---------- ------
$.KEY1     string          8
$.KEY2     string          8
$.KEY3     string         83

This simplified output makes it fast and simple to access the values using the JSON_VALUE function like this.

SELECT JSON_VALUE(js.json_data,'$.KEY1') a,
       JSON_VALUE(js.json_data,'$.KEY2') a,
       JSON_VALUE(js.json_data,'$.KEY3') a
FROM json_stuff js;

Here’s the output.

A          A          A
---------- ---------- ----------
           VALUE2     VALUE3

Note that if JSON_VALUE does not find a KEY it does not fail – it just returns NULL.


Oracle does not require column aliases (all A’s in the example above) to be unique if if they are only for display. This is not exclusive to JSON processing. I only mention it now because it come’s in very handy as we work through this project.

If the aliased column’s are used in subsequent processing it does become a problem as shown here.

  FROM ( SELECT JSON_VALUE(js.json_data,'$.KEY1') a,
                JSON_VALUE(js.json_data,'$.KEY2') a,
                JSON_VALUE(js.json_data,'$.KEY3') a
           FROM json_stuff js );

ERROR at line 1:
ORA-00918: column ambiguously defined

Now that we have JSON data and access to the keys it contains we can start assembling the Polymorphic Table Function (PTF) to process them.

You can find the code for this on the Oracle Live SQL Site.

 Part 1 (This Post) 

Thanks for reading!

The 2020 Project

Way back in the previous decade I asked some PL/SQL developers how they might use Polymorphic Table Functions (PTF) and JSON Dataguides. Almost all of the replies involved translating JSON keys and values into columns and rows on the fly. So with that and a brand new decade in mind I’m embarking on a new series called the 2020 Project.

In a nutshell I’ll be building a PTF to go from this JSON data stored in an Oracle DB.

  FROM json_stuff;

------- ---------------------------------
      1 {"KEY1":"VALUE1"}
      2 {"KEY2":"VALUE2","KEY3":"VALUE3"}

To displaying the keys and values as columns and values like this:

SELECT key1,
  FROM add_dataguide_func(json_stuff);

KEY1   KEY2   KEY3
------ ------ ------
       VALUE2 VALUE3

Over the next few months I’ll assemble and explain code to perform the translation. I’ll post the code on the Oracle livesql site so you can follow along. The introduction code snippets are available here.

NB1 If you just need to qucikly get translations done then have a look a Oracle’s Dataguide indexes and views created by the DBMS_JSON package.

NB2 If you are looking for an in depth tour of PTF’s and JSON Dataguides then I think you’ll enjoy this series.

Thanks for reading!