work_mem vs shared_buffers

In PostgreSQL, work_mem and shared_buffers are two important configuration parameters that control memory usage, but they serve different purposes. Here’s a breakdown of their differences:

1. Purpose

• work_mem:
  • Controls the amount of memory used for per-operation sorting and hashing (e.g., ORDER BY, DISTINCT, GROUP BY, and joins).
  • It is allocated per operation per session, meaning multiple operations in a single query can each use up to work_mem of memory.
  • If the operation requires more memory than work_mem, PostgreSQL will spill to disk (e.g., use temporary files), which can slow down performance.
• shared_buffers:
  • Controls the amount of memory used for caching data pages in shared memory.
  • It is a global memory pool shared across all sessions and is used to cache frequently accessed data from disk.
  • A larger shared_buffers setting can reduce the need to read data from disk, improving performance.

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2. Scope

• work_mem:
  • Per-operation and per-session: Each operation (e.g., sort, hash) in a query can use up to work_mem of memory. If a query has multiple operations, each can allocate its own work_mem.
• shared_buffers:
  • Global: Shared across all sessions and processes. It is used to cache data pages for all queries and operations.

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3. Impact on Performance

• work_mem:
  • Increasing work_mem can improve the performance of queries that involve sorting, hashing, or joining large datasets, as it reduces the need to spill to disk.
  • However, setting it too high can lead to excessive memory usage, especially if there are many concurrent sessions or complex queries.
• shared_buffers:
  • Increasing shared_buffers can improve performance by reducing disk I/O, as more data can be cached in memory.
  • However, setting it too high can leave less memory for other parts of the system (e.g., the operating system’s disk cache).

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4. Default Values

• work_mem:
  • Default is typically 4MB.
  • Can be adjusted dynamically per session or query.
• shared_buffers:
  • Default is typically 128MB.
  • Requires a PostgreSQL restart to change.

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5. When to Adjust

• work_mem:
  • Increase if you notice disk spills (e.g., temporary files being written during sorts or joins).
  • Monitor memory usage to avoid over-allocating.
• shared_buffers:
  • Increase if your workload involves frequent reads of the same data (e.g., OLTP workloads).
  • Typically set to 25-40% of total system memory for dedicated PostgreSQL servers.

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6. Example Scenarios

• work_mem:
  • A query with a large ORDER BY clause might perform slowly if work_mem is too small, as it spills to disk.
  • Increasing work_mem allows the sort to complete in memory, speeding up the query.
• shared_buffers:
  • A query that repeatedly accesses the same table might perform slowly if shared_buffers is too small, as it has to read from disk each time.
  • Increasing shared_buffers caches the table in memory, reducing disk I/O.

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Summary Table

Parameter	work_mem	shared_buffers
Purpose	Memory for sorting/hashing per operation	Memory for caching data pages globally
Scope	Per operation, per session	Global, shared across all sessions
Default Value	4MB	128MB
Impact	Improves sorting/hashing performance	Reduces disk I/O
When to Adjust	Disk spills during sorts/hashes	Frequent disk reads

By tuning these parameters appropriately, you can optimize PostgreSQL’s performance for your specific workload.