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:

Simple

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

Fast

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 )
FOR JSON PARAMETERS ('DATAGUIDE ON');

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.

DATAGUIDE
-------------------------------------------------
[
{
"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,
       type,
       length
  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
---------- ---------- ----------
VALUE1
           VALUE2     VALUE3

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

Foreshadowing

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.

SELECT *
  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 );

SELECT *
*
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.

 Introduction 
 Part 1 (This Post) 

Thanks for reading!

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