Why do postgres DELETEs take up space?

Question

My postgres DB has 20 GB of storage space available but when running a DELETE the space decreases to ~4 GB before increasing again to ~14 GB. I understand that deletions will not return reclaimed space because of the MVCC (want to eventually run a FULL VACUUM on that table) but am confused as to why it went down to ~14 GB from 20 GB ultimately. Additionally, the auto-vacuum did run and was assuming this is why it went down to ~4GB first then went back up..still the return of available storage space happened a good amount of time after the auto vacuum ran so is this an incorrect assumption? I would've assumed some space may still be taken from the DELETEs but 6GB was more than I was expecting.

Steps

1. postgres DB on AWS has 20 GB of available storage space
2. Ran DELETE
3. Storage space decreases to ~4 GB
4. Auto Vacuum runs
5. Time delay but some time after returns to ~14 GB

Unsure why it only returned to 14 GB instead of 20 GB.

Answer 1

DELETE generates WAL, possibly large amounts of WAL. How long this WAL sticks around will depend on your settings and archiving and slots.