On Monday, December 20, 2021, Lars Bergeson <larsava...@gmail.com> wrote:
> > What is it about null values in the table that slows down the full table > scan? > > If I populate blank/zero for all of the unused values in columns that are > NULLable, the query is fast again. So just defining the columns as NULLable > isn't what slows it down -- it's actually the NULL values in the rows that > seems to degrade performance. > The presence or absence of the constraint has zero effect on the contents of the page/tuple. As soon as you have a single null in a row you are adding a null bitmap [1] to the stored tuple. And now for every single column the system has to check whether a specific column’s value is null or not. Given the number of columns in your table, that this is noticeable is not surprising. David J. [1] https://www.postgresql.org/docs/current/storage-page-layout.html
