ORA-64359: INMEMORY clause may not be specified for virtual columns

The title above gives you immediate knowledge of the end result of this post. If you want to know the whys and wherefores then by all means read on.

I was on a call yesterday listening to a presentation on Database 12c new features. The In-Memory Column Store received a lot of interest, as did the new JSON functionality.

One person on the call asked an interesting question, it was something along the lines of:

“If we have a table containing a JSON document can we use the In-Memory Column Store to optimise reports on the JSON attributes”

After some discussion it was decided that the best way to do that is to store the required JSON attributes in dedicated regular columns and report on those. After the call I thought about this some more and wondered if we can:

a) Expose JSON attributes as virtual columns and…
b) Utilise the In-Memory Column Store to report on those virtual columns

I thought these tests were worth executing. Here they are:

a) Can we expose JSON attributes as virtual columns

In order to set up the test I create a table with a JSON column and a virtual column using the function JSON_VALUE to retrieve a scalar value for an attribute called “Name”.

SQL> CREATE TABLE json_docs
(   id          NUMBER (10) NOT NULL
,   date_loaded DATE
,   doc         CLOB
    CONSTRAINT ensure_json CHECK (doc IS JSON)
,   doc_name AS (JSON_VALUE(doc,'$.Name'))
);

Table created.

SQL> desc json_docs
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ID                                        NOT NULL NUMBER(10)
 DATE_LOADED                                        DATE
 DOC                                                CLOB
 DOC_NAME                                           VARCHAR2(4000)

Looks promising. Let’s add some data.

insert into json_docs (id, date_loaded, doc)
values (1,sysdate,'{"Name": "Neil", "Email": "neil@email.com", "Job": "DBA"}');
insert into json_docs (id, date_loaded, doc)
values (2,sysdate,'{"name": "Bob", "Email": "bobl@email.com", "Job": "DBA"}');
commit;

set lines 120
column doc format a60
column doc_name format a10
select * from json_docs;

 ID DATE_LOAD DOC                                                          DOC_NAME
--- --------- ------------------------------------------------------------ --------
  1 13-NOV-14 {"Name": "Neil", "Email": "neil@email.com", "Job": "DBA"}    Neil
  2 13-NOV-14 {"name": "Bob", "Email": "bobl@email.com", "Job": "DBA"}

And there we have it. When the attribute “Name” is present at the top level of the JSON document then we can show it as a dedicated virtual column. I’m not suggesting this is a good idea but… it works.

b) Can we utilise the In-Memory Column Store to report on virtual columns

First I’ll add more data to the JSON_DOCS table.

delete json_docs;
insert into json_docs (id,date_loaded,doc)
select rownum, sysdate, '{"Name": "'||object_name||'", "Type": "'||object_type||'"}'
from all_objects
where rownum <= 500;
commit;

And set the table to be INMEMORY, excluding the JSON document column.

SQL> show parameter inmemory_size

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
inmemory_size                        big integer 252M

SQL> ALTER TABLE json_docs INMEMORY NO INMEMORY(doc);

Next query the data to kick of a load to the “In-Memory Area”

select count(*) from json_docs;
column segment_name format a30
select segment_name,populate_status,bytes_not_populated from v$im_segments order by 1;

SEGMENT_NAME                   POPULATE_ BYTES_NOT_POPULATED
------------------------------ --------- -------------------
JSON_DOCS                      COMPLETED                   0

That looks promising. So let’s check which columns are cached.

select TABLE_NAME,COLUMN_NAME,INMEMORY_COMPRESSION from V$IM_COLUMN_LEVEL;

TABLE_NAME      COLUMN_NAME      INMEMORY_COMPRESSION
--------------- ---------------- ---------------------
JSON_DOCS       ID               DEFAULT
JSON_DOCS       DATE_LOADED      DEFAULT
JSON_DOCS       DOC              NO INMEMORY

Not the virtual column… Can we force it:

ALTER TABLE json_docs INMEMORY (date_loaded, doc_name) NO INMEMORY (doc) ;

*
ERROR at line 1:
ORA-64359: INMEMORY clause may not be specified for virtual columns

Nope. And probably for good reasons.

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