When creating the table for the first time, try using the PCTFREE clause of CREATE TABLE to avoid this issue.

If you create a table like this:

CREATE TABLE example(pk bigint, col1 int, col2 char(4));

the row width is 16 bytes. There is a small amount of overhead for each row, as well as a small amount of overhead for each page.

We'll fill the page with rows:

INSERT INTO example
SELECT row_num, 1, 'abcd' FROM sa_rowgenerator(1,168);
CHECKPOINT;
SELECT table_page_count,ext_page_count FROM SYSTABLE WHERE table_name = 'example';

Result:1,0

Notice exactly one page is used. Now add a column:

ALTER TABLE example ADD newcol int;
UPDATE example SET newcol = 2;
CHECKPOINT;
SELECT table_page_count,ext_page_count FROM SYSTABLE WHERE table_name = 'example';

Result:1,1

Because the page holding the row was already full, the new column had to go in a separate extended page. Now a SELECT pk, newcol FROM example has to read from two pages for the rows in question. This is slower than having the whole row on the same page.

If we add PCTFREE 15 to reserve space in each page, columns can be added with more flexibility. Of course this also mean that we'll have less rows per page, and inefficient page usage if the columns are never added.

-- Create table with 15% free reserved space
DROP TABLE example;
CREATE TABLE example  (pk bigint, col1 int, col2 char(4), PCTFREE 15) ;

-- Fill page (the page holds less rows now, because we are reserving 15%)
INSERT INTO example
SELECT row_num, 1, 'abcd' FROM sa_rowgenerator(1,150);
CHECKPOINT;
SELECT table_page_count,ext_page_count FROM SYSTABLE WHERE table_name = 'example';

-- Add new column
ALTER TABLE example ADD newcol int;
UPDATE example SET newcol = 2;

-- Observe that the new column fits on the same page
CHECKPOINT;
SELECT table_page_count,ext_page_count FROM SYSTABLE WHERE table_name = 'example';

Result:1,0