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  3. April 2024

Re: Db2 and clob columns

lastly,

when testing...

is the best way to confirm no padded spaces being stored is to...

clear table
run a few transactions to store the rows
divide table size by row count?

(should not show storage up to 10mb (for the 2 5mb clob columns))... right?

Jay

On Tue, Apr 16, 2024 at 12:08 PM Jon Paris <jon.paris@xxxxxxxxxxxxxx> wrote:

You missed the

g_clob_data = i_5000000;

bit. But otherwise yes.


Jon P.

On Apr 16, 2024, at 11:12 AM, Jay Vaughn <jeffersonvaughn@xxxxxxxxx>
wrote:

ok that is nearly the code I already had with the exception of...

i_5000000 varchar(5242880) const;

and

g_clob_len = %len(i_5000000);

Jon, are you saying this is really the only change I really need?
Just wanting to confirm.

thanks

Jay



On Tue, Apr 16, 2024 at 11:02 AM Jon Paris <jon.paris@xxxxxxxxxxxxxx>
wrote:

As Charles has pointed out the Trims here are pretty much a waste of
time
if the varchar was loaded correctly (i.e. via a %TrimR).

Also in the rare event that the caller _wanted_ trailing spaces and had
set the field up accordingly then this code would remove them.

dcl-pi
i_5000000 varchar(5242880) CONST;
end-pi;

dcl-s g_clob sqltype(clob:5242880) static;

g_clob_data = %TRIMR(i_5000000);
g_clob_len = %len(%TRIMR(i_5000000));

In fact the code should just be:

dcl-pi
i_5000000 varchar(5242880) CONST;
end-pi;

dcl-s g_clob sqltype(clob:5242880) static;

g_clob_data = i_5000000; // Trim is pointless as the compiler will just
add back blanks to pad g_clob_data anyway
g_clob_len = %len(i_5000000); // Again the trim does nothing except
waste time


Jon P.

On Apr 16, 2024, at 10:50 AM, Charles Wilt <charles.wilt@xxxxxxxxx>
wrote:

Thinking about it...

While the %TRIMR() should be a no-op if the data is already trimmed...
I don't think %LEN(%TRIMR()) would be...

Depending on how often this is called, I might get rid of the
%TRIMR()...or
only do them if the %LEN() is larger than the 64k expected.

But the change to CONST and the use of STATIC will greatly increase the
performance compared to the original.

Charles

On Tue, Apr 16, 2024 at 8:21 AM Charles Wilt <charles.wilt@xxxxxxxxx>
wrote:

Since g_clob_data is CHAR(5242880)

The %LEN(g_clob_data) is always 5242880, so yep you're writing out
blanks.

You should use %LEN(i_5000000);

But even there, if the data is passed in with trailing blanks, then
yep
you're getting 5MB written out.

A quick fix would be to add OPTIONS(*TRIM) to the parameter, but that
would require a recompile of callers.

You could also add %TRIMR(), but you'd end up doing so twice...

Personally, I'd prefer to ensure the data is trimmed at the origin and
then pass in as CONST instead of VALUE instead of copying the value to
the
stack.
Also, I'd define g_clob as STATIC, to keep from having it created and
deleted (and INZ) for every call.

I might take a boots and suspenders approach, including a %TRIMR() ...
if
the data is already trimmed it should be a no-op.

dcl-pi
i_5000000 varchar(5242880) CONST;
end-pi;

dcl-s g_clob sqltype(clob:5242880) static;

g_clob_data = %TRIMR(i_5000000);
g_clob_len = %len(%TRIMR(i_5000000));

insert into table(myClob)
values (:g_clob);


Charles


On Tue, Apr 16, 2024 at 7:38 AM Jay Vaughn <jeffersonvaughn@xxxxxxxxx

wrote:

we have a procedure...

dcl-pi
i_5000000 varchar(5242880) value;
end-pi;

dcl-s g_clob sqltype(clob:5242880) inz;

g_clob_data = i_5000000;
g_clob_len = %len(g_clob_data);

insert into table(myClob)
values (:g_clob);


thoughts?

Jay







On Tue, Apr 16, 2024 at 9:31 AM Charles Wilt <charles.wilt@xxxxxxxxx

wrote:

If you have RPG code writing out the table, I'd look there first.

Make sure the RPG code is properly handling the varchar data.

Charles

On Tue, Apr 16, 2024 at 7:00 AM Jay Vaughn <
jeffersonvaughn@xxxxxxxxx

wrote:

So Charles,

We have a table developed to hold a request clob (5mb) and a
response
clob
(5mb) ONLY for storage and auditing purposes.
Not to run queries against.

We have been finding out that some off platform load tests are
being
performed and even though the api calls to put rows in this table
should
have 1mb or less requests/responses, we are maxing out the DASD on
that
test box and this table ends up with 821G of data.

Again our requests/responses should be well under 1mb each.

So is this table just a bad idea or are we simply over dominating
the
box
with our load test volume?

Next time this happens I really want to look at the rows and see
what
these
columns have in them.

thanks

Jay








On Mon, Apr 15, 2024 at 6:51 PM Jay Vaughn <
jeffersonvaughn@xxxxxxxxx

wrote:

Thanks Charles.

I’lllook into the allocate but big relief on the storage concern.

Jay

On Apr 15, 2024, at 5:44 PM, Charles Wilt <
charles.wilt@xxxxxxxxx

wrote:

Nope...

variable length data is stored in the overflow section of the
table...

Downside of that is that when you read the row, it takes 2 I/Os.
One
for
the row space and one for the overflow.

If you were mostly writing less that 32k, I'd tell you to take a
look
at
the ALLOCATE clause. That does cause space to be reserved in the
standard
table space. Which wastes space, but then the data only needs 1
I/O
to
be
read.

Charles


On Mon, Apr 15, 2024 at 2:20 PM Jay Vaughn <
jeffersonvaughn@xxxxxxxxx

wrote:

Let’s say I have a table with a 5mb clob column.

Most of my transactions that write to the table mostly consist
of
64k
and
less chars written to the clob but we want to reserve the much
higher
storage allocation for when we need it

When the 64k chars are written to that clob on the row, the
entire
5mb
of
storage is not written to disk is it??

Tia

Jay

Sent from my iPhone
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