Re: Db2 and clob columns

fabulous Charles - thank you

Jay

On Tue, Apr 16, 2024 at 11:37 AM Charles Wilt <charles.wilt@xxxxxxxxx>
wrote:

If the data is already trimmed,
%len(%trimr(i_5000000))

Wastes time re-calculating the size...at least I think it would. I'd

have

to check to make sure.
%len() of a VARCHAR field just returns the current length value stored in
the initial (2 or 4) bytes.

So only if it's larger than normally expected, would I do the trim.

OTOH, if as you said you never want to store the padding, then just
g_clob_len = %len(%trimr(i_5000000));

The wasted time might not be enough to matter, especially if you'd rather
waste time than space.

Charles



On Tue, Apr 16, 2024 at 9:20 AM Jay Vaughn <jeffersonvaughn@xxxxxxxxx>
wrote:

I don't understand that code Charles...
why not trim if <= 64000?


I'd probably do
if %len(i_5000000) <= 64000;
g_clob_len = %len(i_5000000);
else;
g_clob_len = %len(%trimr(i_5000000));
endif;

On Tue, Apr 16, 2024 at 11:17 AM Jay Vaughn <jeffersonvaughn@xxxxxxxxx

wrote:

correct - in no situation do we want padded blanks

Jay

On Tue, Apr 16, 2024 at 11:16 AM Charles Wilt <

charles.wilt@xxxxxxxxx>

wrote:

Jon's correct in that I overthought this as far as g_clob_data..

This is indeed all that should be there..
g_clob_data = i_5000000;

However, I'd argue about g_glob_len...

Given Jay's description of the purpose, I'd argue that allowing the

caller

to include blanks is not valid and a waste of space and that I as

the

owner
of the procedure do not want to allow that.

I'd probably do
if %len(i_5000000) <= 64000;
g_clob_len = %len(i_5000000);
else;
g_clob_len = %len(%trimr(i_5000000));
endif;

Charles

On Tue, Apr 16, 2024 at 9:02 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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