Lars Bergeson <larsavatar@xxxxxxxxx> writes: > What is it about null values in the table that slows down the full table > scan? If a row has any nulls, then it contains a "nulls bitmap" [1] that says which columns are null, and that bitmap has to be consulted while walking through the row contents. So the most obvious theory here is that that adds overhead that's significant in your case. But there are some holes in that theory, mainly that the I/O timings you are showing don't seem very consistent: no nulls: > I/O Timings: read=1884365.335 > Execution Time: 11135.368 ms with nulls: > I/O Timings: read=17141420.771 > Execution Time: 25407.318 ms Regardless of CPU time required, it should not take 10X less I/O time to read a physically larger table. So there's something fairly bogus going on there. One thing you might try is disabling parallelism (set max_parallel_workers_per_gather = 0) to see if that's confusing the numbers somehow. regards, tom lane [1] https://www.postgresql.org/docs/current/storage-page-layout.html#STORAGE-TUPLE-LAYOUT
