iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
PostgreSQL temporary tables do not have a universal 1,024-row or 1,024-block limit. A PostgreSQL 18 report describes a narrower failure: under particular scan and I/O conditions, ReadStream look-ahead pinned all 1,024 local buffers in the reproducer, then fetching a TOAST value required one more buffer and failed. The report is conditional, not proof that ordinary temporary-table scans—or every PostgreSQL release—will fail this way.
What does temp_buffers control?
temp_buffers sets the maximum memory available for temporary-table buffers in each database session. PostgreSQL 18 documentation lists an 8MB default. The buffers hold temporary-table pages; the setting is not a count of SQL counters, and it does not cap a table’s row count or size. PostgreSQL allocates buffers as needed up to the configured maximum. PostgreSQL 18 resource configuration documentation.
A session can change temp_buffers only before its first use of a temporary table. Once a temporary table has been used in that session, later attempts to change the setting have no effect there. This is a per-session setting, so changing it does not retroactively alter sessions already using temporary tables.
Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteWhat happened in the reported PostgreSQL 18 case?
In a PostgreSQL mailing-list report dated July 3, 2026, Xuneng Zhou described a reproducer in which ReadStream scan look-ahead could pin every local buffer in the default 1,024-buffer pool. With io_combine_limit=16, the report says an effective_io_concurrency value of 64 or higher made the look-ahead formula exceed that pool. Xuneng Zhou’s PostgreSQL mailing-list report.
#1 Best Overall
Why a further buffer was needed
The reproducer used a temporary table of roughly 1,333 heap blocks, larger than the 1,024-buffer pool. During a cold-miss scan, look-ahead filled the window. The output also included a TOASTed column: retrieving its out-of-line value required another buffer. According to Zhou’s account, that request failed because all 1,024 local buffers were pinned.
The table size alone does not trigger the reported condition. The described failure depends on the interaction of look-ahead, a cold-miss scan, and an additional buffer request for TOAST data. Zhou noted that the two conditions coinciding may be difficult to guarantee in a changing production workload. The report therefore describes a conditional failure mode, not a general size threshold for temporary tables.
Rank #2
How this differs from work_mem and temp_file_limit
These settings govern different resources. Temporary-table buffers are not the same thing as memory used by query operations or temporary files written when operations spill.
Recommended Free Tools
| Setting | Scope | What it governs | At the limit |
|---|---|---|---|
temp_buffers |
Each database session | Buffers used for temporary-table pages; PostgreSQL 18 documents an 8MB default. | Temporary-table buffer capacity is bounded by the configured maximum. The reported failure involved a request when all local buffers were pinned. |
work_mem |
Each query operation, such as a sort or hash table | Memory an operation can use before writing temporary files. Several operations in a query and concurrent sessions can each consume memory under this setting; hash operations are also affected by hash_mem_multiplier. |
An operation may write temporary files rather than keep exceeding its memory allowance. See the PostgreSQL 17 resource configuration documentation. |
temp_file_limit |
Each process | Size of certain temporary files, including sort/hash files and held-cursor storage. Explicit temporary-table storage is excluded. | Limits covered temporary files; it does not limit storage occupied by an explicitly created temporary table. See the PostgreSQL 17 resource configuration documentation. |
Does increasing a setting fix the reported failure?
The cited report and documentation do not establish a generally effective tuning fix or confirm the issue’s fix status across PostgreSQL releases. Increasing work_mem addresses a different resource from temporary-table buffers; temp_file_limit also does not cap explicit temporary-table storage. Although temp_buffers controls the relevant pool and can be set before a session first uses a temporary table, the available evidence does not test a configuration change as a remedy for this particular interaction.
The report concerns a PostgreSQL 18 reproducer. It does not establish that other releases are affected, that every PostgreSQL 18 workload is vulnerable, or that the behavior remains present in later builds. Treat the 1,024 figure as the pool size in that reported setup—not a universal temporary-table limit.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
