Re: TOAST condition for column size - Mailing list pgsql-hackers

On Tue, Jan 19, 2021 at 5:18 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Mon, Jan 18, 2021 at 7:53 PM torikoshia <torikoshia@oss.nttdata.com> wrote:
> >
> > Hi,
> >
> > When I created a table consisting of 400 VARCHAR columns and tried
> > to INSERT a record which rows were all the same size, there were
> > cases where I got an error due to exceeding the size limit per
> > row.
> >
> >    =# -- create a table consisting of 400 VARCHAR columns
> >    =# CREATE TABLE t1 (c1 VARCHAR(100),
> >                        c2 VARCHAR(100),
> >                        ...
> >                        c400 VARCHAR(100));
> >
> >    =# -- insert one record which rows are all 20 bytes
> >    =# INSERT INTO t1 VALUES (repeat('a', 20),
> >                              repeat('a', 20),
> >                              ...
> >                              repeat('a', 20));
> >      ERROR:  row is too big: size 8424, maximum size 8160
> >
> > What is interesting is that it failed only when the size of each
> > column was 20~23 bytes, as shown below.
> >
> >    size of each column  |  result
> >    -------------------------------
> >            18 bytes     |  success
> >            19 bytes     |  success
> >            20 bytes     |  failure
> >            21 bytes     |  failure
> >            22 bytes     |  failure
> >            23 bytes     |  failure
> >            24 bytes     |  success
> >            25 bytes     |  success
> >
> >
> > When the size of each column was 19 bytes or less, it succeeds
> > because the row size is within a page size.
> > When the size of each column was 24 bytes or more, it also
> > succeeds because columns are TOASTed and the row size is reduced
> > to less than one page size.
> > OTOH, when it's more than 19 bytes and less than 24 bytes,
> > columns aren't TOASTed because it doesn't meet the condition of
> > the following if statement.
> >
> >   --src/backend/access/table/toast_helper.c
> >
> >     toast_tuple_find_biggest_attribute(ToastTupleContext *ttc,
> >                           bool for_compression, bool check_main)
> >         ...(snip)...
> >         int32        biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
> >         ...(snip)...
> >         if (ttc->ttc_attr[i].tai_size > biggest_size) // <- here
> >         {
> >             biggest_attno = i;
> >             biggest_size = ttc->ttc_attr[i].tai_size;
> >         }
> >
> >
> > Since TOAST_POINTER_SIZE is 18 bytes but
> > MAXALIGN(TOAST_POINTER_SIZE) is 24 bytes, columns are not TOASTed
> > until its size becomes larger than 24 bytes.
> >
> > I confirmed these sizes in my environment but AFAIU they would be
> > the same size in any environment.
> >
> > So, as a result of adjusting the alignment, 20~23 bytes seems to
> > fail.
> >
> > I wonder if it might be better not to adjust the alignment here
> > as an attached patch because it succeeded in inserting 20~23
> > bytes records.
> > Or is there reasons to add the alignment here?
> >
> > I understand that TOAST is not effective for small data and it's
> > not recommended to create a table containing hundreds of columns,
> > but I think cases that can be successful should be successful.
> >
> > Any thoughts?
>
> How this can be correct? because while forming the tuple you might
> need the alignment.
>

Won't it be safe because we don't align individual attrs of type
varchar where length is less than equal to 127?

-- 
With Regards,
Amit Kapila.