PostgreSQL’s configured connection limit is set by max_connections. In PostgreSQL 18, the documented default is typically 100 concurrent connections, but that is an admission limit—not a promise that every workload will perform well with 100 active queries. The number a server can handle efficiently depends on its resources and workload; for many client sessions, pooling can be preferable to raising the server limit.
Three different meanings of “handle”
The question can refer to three different capacities, and they should not be treated as interchangeable:
- Concurrent database sessions: How many connections PostgreSQL is configured to admit. The PostgreSQL 18 connection settings define this limit through
max_connections. - Simultaneous useful work: How many queries or transactions the server can execute efficiently before CPU, memory, storage, or other resource contention reduces throughput.
- Application clients served: How many application clients can use the database through a connection pool or another session-management layer. That count can exceed the number of backend connections active at once.
So PostgreSQL has a configurable session cap, but there is no universal number of active queries that is safe or optimal for every installation.
What is PostgreSQL’s default connection limit?
The PostgreSQL 18 manual says max_connections is typically 100 by default. The value is configurable and may be lower if kernel settings do not support the default. Check the deployed server rather than assuming it uses the documented typical value.
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The parameter sets the maximum number of concurrent connections the server permits. PostgreSQL also allocates some resources based on this setting; the official documentation warns that increasing it increases allocation, including shared memory. It does not establish a universal per-connection memory cost, so avoid estimating total memory by multiplying the limit by a single assumed figure.
Why the configured maximum is not a performance target
Permitting more connections does not necessarily let PostgreSQL complete more work per second. Additional active sessions compete for finite server resources. More concurrent work may help up to a point, but after saturation, contention can reduce throughput and increase waiting.
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The PostgreSQL Wiki discussion of connection counts describes this general trade-off. Its tuning guidance says good hardware may support a few hundred connections and suggests considering pooling for workloads targeting thousands. These are broad community observations, not measured limits or guarantees for a particular server.
The same wiki page mentions an older rule of thumb based on CPU cores and effective disk spindles, while noting that the formula needs adjustment across versions and does not analyze SSD performance. It is not a sound modern sizing formula. Measure the workload you actually run instead.
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Reserved slots and who can use them
Some slots may be reserved rather than available to ordinary connection attempts. In PostgreSQL 18, the documented defaults are three superuser_reserved_connections and zero reserved_connections. The latter are available to roles granted pg_use_reserved_connections; the final superuser-reserved slots are for superusers. Both settings are constrained by max_connections.
These reservations affect admission when the server approaches its cap: an ordinary application role may be refused a connection even though reserved slots remain. They do not increase the configured maximum.
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How to choose a connection limit
There is no exact safe number without details about the server, queries, and traffic pattern. Size the limit from observed use and resource behavior, not from the number of application users or clients alone.
- Inspect the deployed setting. Check the active
max_connectionsvalue and the reserved-slot settings on the server. Do not infer them from defaults, since configuration and platform constraints can change the effective limit. - Measure connection use. Track total, active, and idle sessions over normal periods and traffic peaks. Identify whether connections are performing work or mostly waiting.
- Look for resource pressure and query behavior. Observe memory pressure, CPU and storage utilization, query latency, and throughput as concurrency rises. PostgreSQL memory use depends on configuration and workload; do not assume each connection always consumes a fixed amount such as
work_mem. - Adjust incrementally and test representative traffic. If evidence shows the server needs more concurrent sessions and has capacity, raise the limit in measured increments and compare results. If contention worsens or memory becomes pressured, increasing the cap may be counterproductive.
- Use a pool when client counts exceed useful backend concurrency. A pool can limit active database connections and queue work rather than sending every client directly to PostgreSQL. Reassess its queueing and latency behavior under bursts.
For memory pressure, the official connection settings guidance recognizes that reducing max_connections and using external pooling may be preferable. The PostgreSQL 18 resource settings documentation gives separate guidance for shared_buffers: it is typically 128 MB by default, and 25% of system memory is offered as a reasonable starting point for a dedicated database server with at least 1 GB of RAM. That is general buffer configuration advice, not a formula for connection capacity.
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Direct connections or pooling?
| Approach | Client sessions versus backend sessions | Bursts and waiting | Database resource pressure |
|---|---|---|---|
| Direct connections | Each client connection uses a PostgreSQL connection; the configured cap limits admitted concurrent sessions. | Clients compete for connections when demand approaches the cap; behavior depends on the application and its retry or queueing strategy. | More active sessions can increase contention. The limit itself also affects allocation of some resources. |
| External connection pooling | A pool can serve many client sessions through a smaller number of active PostgreSQL backend connections. | The pool can queue work when backend capacity is occupied, which can manage bursts but may add waiting and latency. | Capping active backends can reduce pressure compared with allowing every client to connect directly; suitable pool sizing still depends on the workload. |
Pooling is not identical to keeping persistent connections open: persistent connections alone do not pool work. Pool modes can also differ in application compatibility and transaction or session semantics, so those details must be checked for the specific pool implementation. The PostgreSQL Wiki connection-pooling overview discusses pooling as an option for managing high client counts.
Replication and standby connection limits
If queries are allowed on a standby, its max_connections must be at least as high as the primary’s. The PostgreSQL documentation on hot standby describes this requirement. A primary and standby therefore should not be configured independently without checking this constraint.
How to change the limit
max_connections can only be set at server start, so changing it requires a server restart. Consult the documentation and administration procedure for the PostgreSQL installation or managed service in use; restart controls vary by environment. Before raising the value, account for the increased resource allocation and confirm that the workload benefits from more simultaneous backend sessions.
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