IO:BuffileWrite
This waitevent occurs when PostgreSQL needs to write data to temporary files on disk as part of SQL execution.
This typically happens when operations require more memory than the work_mem parameter allows, causing the system to spill data to disk.
These temporary files are used internally by PostgreSQL for operations that exceed available memory, such as large sorts, hash joins, materialized intermediate results, or batched aggregations.
From SQL tuning perspective, We need to check whether large amount of data is pulled into memory for sort and join operations. Good filtering conditions are important.
For Further reading: [IO:BufFileRead and IO:BufFileWrite]
Common Causes
Temporary files are created when large volume of data is fetched for operations like sorting, joining, or aggregating that exceed the allocated memory (work_mem).
- heavy SQLSQL statement will be fetching huge volume of data from tables and indexes for processing
- Large sorts: ORDER BY, DISTINCT, GROUP BY clauses on large datasets
- Large joins: JOIN operations involving large tables without proper indexing
- Hash Joins and Aggregations: Operations like SUM, AVG, COUNT on large datasets
- Creation of Large Indexes: Building a B-tree index is essentially a massive sort operation. If the index size is larger than maintenance_work_mem, the CREATE INDEX or REINDEX command will write temporary files to the disk.
- Window Functions: Complex window functions (e.g., RANK(), OVER(PARTITION BY...)) often require large-scale sorting or buffering of row sets that can easily exceed memory limits.
- Insufficient work_mem: If the work_mem parameter is set too low, PostgreSQL will spill more data to temporary files, increasing I/O overhead. Set work_mem at lowest scope possible