IO:BufFileRead

BufFileRead happens when temporary files generated for SQL execution are read back to memory.
Generally this happens after BuffileWrite.
BufFileRead means that PostgreSQL is Reading from buffered Temporary Files.
All sorts of temporary files including the one used for sort and hashjoins, parallel execution, And files used by single sessions (refer: buffile.c) can be responsible for this.
Query tuning effort is suggestable.