diff --git a/internal/app/services.go b/internal/app/services.go
index a9743925..a40c76fd 100644
--- a/internal/app/services.go
+++ b/internal/app/services.go
@@ -5,6 +5,8 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/compose-spec/compose-go/v2/types"
+
 	"github.com/labstack/onebox/internal/shellquote"
 )
 
@@ -466,14 +468,19 @@ func (p *Spec) renderService(n Names, name string, s Service, selectedImage stri
 			"labels": map[string]any{"onebox.app": p.Name, "onebox.service": name},
 		}
 	}
+	memory := ""
 	if s.Resources != nil {
-		if s.Resources.Memory != "" {
-			svc["mem_limit"] = s.Resources.Memory
+		memory = s.Resources.Memory
+		if memory != "" {
+			svc["mem_limit"] = memory
 		}
 		if s.Resources.CPUs != "" {
 			svc["cpus"] = s.Resources.CPUs
 		}
 	}
+	if key == "postgres" {
+		svc["shm_size"] = postgresShmSize(memory)
+	}
 
 	// The whole document is generated, and the shell expansions in it are meant
 	// for the container, not for Compose on the host.
@@ -495,6 +502,32 @@ func (p *Spec) renderService(n Names, name string, s Service, selectedImage stri
 	return append([]byte(header), body...), nil
 }
 
+// postgresShmSize sets the POSIX shared-memory ceiling in /dev/shm for parallel
+// queries. Docker's default 64 MiB is easily exhausted. This ceiling allocates
+// nothing up front, but used pages count against mem_limit, so even the floor
+// stays within half.
+func postgresShmSize(memory string) int64 {
+	if memory == "" {
+		return 1 << 30
+	}
+	var limit types.UnitBytes
+	// Use Compose's memory parser. Zero means unlimited; negative means overflow.
+	// Leave mem_limit untouched for existing validation.
+	if err := limit.DecodeMapstructure(memory); err != nil || limit <= 0 {
+		return 1 << 30
+	}
+	shmSize := int64(limit) / 4
+	if shmSize < 256<<20 {
+		shmSize = 256 << 20
+	} else if shmSize > 1<<30 {
+		shmSize = 1 << 30
+	}
+	if shmSize > int64(limit)/2 {
+		shmSize = int64(limit) / 2
+	}
+	return shmSize
+}
+
 // identityEnv is the user and database the service is created with, under the
 // variable names each driver expects. The user is the driver's fixed role and
 // the database is named after the application.
diff --git a/internal/app/services_test.go b/internal/app/services_test.go
index cd71ff25..74dd1af4 100644
--- a/internal/app/services_test.go
+++ b/internal/app/services_test.go
@@ -6,6 +6,8 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+
+	"gopkg.in/yaml.v3"
 )
 
 func serviceSpec(t *testing.T, body string) *Spec {
@@ -65,6 +67,67 @@ func TestServiceDocumentCarriesNoCredential(t *testing.T) {
 	}
 }
 
+func TestPostgresSharedMemory(t *testing.T) {
+	for _, tt := range []struct {
+		name, version, resources, wantMemLimit string
+		want                                   int64
+	}{
+		{"default", "18", "", "", 1 << 30},
+		{"official image", "17", "", "", 1 << 30},
+		{"empty resources", "18", ", resources: {}", "", 1 << 30},
+		{"CPU limit only", "18", ", resources: {cpus: '0.5'}", "", 1 << 30},
+		{"byte limit", "18", ", resources: {memory: 1B}", "1B", 0},
+		{"128 MiB limit", "18", ", resources: {memory: 128MB}", "128MB", 64 << 20},
+		{"256 MiB limit", "18", ", resources: {memory: 256MB}", "256MB", 128 << 20},
+		{"small limit", "18", ", resources: {memory: 512MB}", "512MB", 256 << 20},
+		{"zero limit", "18", ", resources: {memory: 0B}", "0B", 1 << 30},
+		{"floor boundary", "18", ", resources: {memory: 1GB}", "1GB", 256 << 20},
+		{"mid limit", "18", ", resources: {memory: 2GB}", "2GB", 512 << 20},
+		{"decimal limit", "18", ", resources: {memory: 1.5GB}", "1.5GB", 384 << 20},
+		{"cap boundary", "18", ", resources: {memory: 4GB}", "4GB", 1 << 30},
+		{"large limit", "18", ", resources: {memory: 8GB}", "8GB", 1 << 30},
+		// The size grammar accepts this, but it overflows Compose's integer limit.
+		// Rendering must preserve mem_limit for the existing validation path.
+		{"overflowing limit", "18", ", resources: {memory: 999999999999999999999999TB}", "999999999999999999999999TB", 1 << 30},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			doc := renderStore(t, "services: {store: {driver: postgres, version: "+tt.version+tt.resources+"}}\n")
+			var runtime struct {
+				Services map[string]struct {
+					ShmSize  int64  `yaml:"shm_size"`
+					MemLimit string `yaml:"mem_limit"`
+				} `yaml:"services"`
+			}
+			if err := yaml.Unmarshal([]byte(doc), &runtime); err != nil {
+				t.Fatal(err)
+			}
+			service := runtime.Services["store"]
+			if service.ShmSize != tt.want {
+				t.Fatalf("shm_size = %d, want %d:\n%s", service.ShmSize, tt.want, doc)
+			}
+			if service.MemLimit != tt.wantMemLimit {
+				t.Fatalf("mem_limit = %q, want %q", service.MemLimit, tt.wantMemLimit)
+			}
+		})
+	}
+}
+
+func TestOtherServicesHaveNoSharedMemoryOverride(t *testing.T) {
+	for _, driver := range DriverNames() {
+		if driver == "postgres" {
+			continue
+		}
+		t.Run(driver, func(t *testing.T) {
+			for _, resources := range []string{"", ", resources: {memory: 2GB}"} {
+				doc := renderStore(t, "services: {store: {driver: "+driver+", version: '1'"+resources+"}}\n")
+				if strings.Contains(doc, "shm_size") {
+					t.Fatalf("%s has a shared-memory override:\n%s", driver, doc)
+				}
+			}
+		})
+	}
+}
+
 // A workload that needs a service must be able to reach it and to know how.
 func TestNeedingAServiceJoinsItAndReadsItsURL(t *testing.T) {
 	r, err := serviceSpec(t, "services: {store: {driver: postgres, version: 17}}\n").Resolve("production")
diff --git a/internal/app/testdata/contract-verdicts.json b/internal/app/testdata/contract-verdicts.json
index d6c95be0..8017da16 100644
--- a/internal/app/testdata/contract-verdicts.json
+++ b/internal/app/testdata/contract-verdicts.json
@@ -392,12 +392,12 @@
   {
     "case": "conformance/service backup policy",
     "loads": true,
-    "digest": "73135190e9f9fae42347e107f496211e4d2f75bfe5ff934ead0096f4f3571a19 postgres=ee9972ab51c56731"
+    "digest": "73135190e9f9fae42347e107f496211e4d2f75bfe5ff934ead0096f4f3571a19 postgres=ee21ed4fc49969fb"
   },
   {
     "case": "conformance/service scalar",
     "loads": true,
-    "digest": "f6ddf429bf783de806d4b1475df352d4b3ff85210dfe379804248dd3fdd0b80a postgres=f566c9221cb196ab"
+    "digest": "f6ddf429bf783de806d4b1475df352d4b3ff85210dfe379804248dd3fdd0b80a postgres=ff6eeff3e4bbd9bd"
   },
   {
     "case": "conformance/settings key that is a real driver flag",
@@ -532,12 +532,12 @@
   {
     "case": "corpus/authentik.yml",
     "loads": true,
-    "digest": "e82f2e9eac87e3494cf0aa86b80f91a60f363f0af7aac28bb45e1ec6ed2e40fb postgres=d88fd3d620b30ea1 redis=53bad39e8f359a02"
+    "digest": "e82f2e9eac87e3494cf0aa86b80f91a60f363f0af7aac28bb45e1ec6ed2e40fb postgres=4d380770b261847a redis=53bad39e8f359a02"
   },
   {
     "case": "corpus/ext-authentik-managed.yml",
     "loads": true,
-    "digest": "10748708444a383dc2e587286c13f06a4f3ea218835e2eeab1af83ed312ec741 postgres=d88fd3d620b30ea1 redis=53bad39e8f359a02"
+    "digest": "10748708444a383dc2e587286c13f06a4f3ea218835e2eeab1af83ed312ec741 postgres=4d380770b261847a redis=53bad39e8f359a02"
   },
   {
     "case": "corpus/ext-authentik.yml",
@@ -552,7 +552,7 @@
   {
     "case": "corpus/ext-gitea.yml",
     "loads": true,
-    "digest": "f6028df90e48b43f68c3375115ef402255ebd0aadf0ddde9eb9e2d538598cb65 postgres=446e25618a3d2e4e"
+    "digest": "f6028df90e48b43f68c3375115ef402255ebd0aadf0ddde9eb9e2d538598cb65 postgres=105be1b298f59e44"
   },
   {
     "case": "corpus/ext-immich-sourced.yml",
@@ -567,7 +567,7 @@
   {
     "case": "corpus/ext-n8n.yml",
     "loads": true,
-    "digest": "3831bd9a52d01deada4936e218fc8ba33c96a0b1d939e6bf503f3fbda8ba0895 postgres=c9e06c2c21513656 redis=83f7ec43501bb1f4"
+    "digest": "3831bd9a52d01deada4936e218fc8ba33c96a0b1d939e6bf503f3fbda8ba0895 postgres=12dc8bcbdff40808 redis=83f7ec43501bb1f4"
   },
   {
     "case": "corpus/ext-paperless.yml",
@@ -577,12 +577,12 @@
   {
     "case": "corpus/ext-plausible.yml",
     "loads": true,
-    "digest": "4574810017062b8d857ad2c43597b484fe88e6b4198f75ec74c2d705cbcf17d1 events=79a3b4327069a1dc postgres=128c7ea3ed2b4f69"
+    "digest": "4574810017062b8d857ad2c43597b484fe88e6b4198f75ec74c2d705cbcf17d1 events=79a3b4327069a1dc postgres=1aea911e88a3631c"
   },
   {
     "case": "corpus/ext-umami.yml",
     "loads": true,
-    "digest": "fd193a8c2f51cbe206e2e038ccd277c3e6785c84912b5967a2afd58c1320ac13 postgres=a0f143f02b9f5845"
+    "digest": "fd193a8c2f51cbe206e2e038ccd277c3e6785c84912b5967a2afd58c1320ac13 postgres=47bbd25b517473d9"
   },
   {
     "case": "corpus/ghost.yml",
@@ -622,7 +622,7 @@
   {
     "case": "corpus/penpot.yml",
     "loads": true,
-    "digest": "1cd500aae4deac5cf744dec3f1222b13aec493457de4b6069a3449126ad70210 postgres=fce3c20f4ad2e0e4 redis=e6f4fb5264fe75ae"
+    "digest": "1cd500aae4deac5cf744dec3f1222b13aec493457de4b6069a3449126ad70210 postgres=8a2a89524e0749e0 redis=e6f4fb5264fe75ae"
   },
   {
     "case": "corpus/pursue.yml",
diff --git a/site/src/content/docs/guides/add-a-database.mdx b/site/src/content/docs/guides/add-a-database.mdx
index fc86ff3d..bf91e5fe 100644
--- a/site/src/content/docs/guides/add-a-database.mdx
+++ b/site/src/content/docs/guides/add-a-database.mdx
@@ -23,15 +23,24 @@ That is sufficient as a declaration. Onebox supplies the image, a durable volume
 a health check, a credential generated on the server, and the connection details
 your application reads. You supply no password, no port, and no data path.
 
+PostgreSQL gets a 1 GiB `/dev/shm` ceiling for parallel queries; with a
+positive `resources.memory` limit, it gets one quarter of that limit, bounded
+between 256 MiB and 1 GiB, but never more than half the limit. A zero limit means
+unlimited. This tmpfs uses memory only as PostgreSQL writes to it, and those
+pages count against the container's memory limit.
+
 Declaring it does not start it. Services live outside the release lifecycle, so
-`ob deploy` wires up connections but never creates or replaces a service
-container — that would make every deploy a potential database restart. Apply the
-service explicitly:
+`ob deploy` wires up connections. For services with declared extensions, deploy
+also converges their configuration and recreates containers when it changes.
+Apply a service explicitly with:
 
 ```sh
 ob service apply --output ndjson
 ```
 
+A running PostgreSQL service keeps its current `/dev/shm` ceiling until
+`ob service apply` recreates it, or deploy does for services with declared extensions.
+
 On a host you have not bootstrapped yet, `ob bootstrap` does this for you. If you
 skip it on a live application, the next deploy stops at preflight with
 `service "postgres" is not running — start it with ob bootstrap or ob service apply`.
@@ -165,8 +174,10 @@ is omitted rather than written empty.
 
 ## Three properties that always hold
 
-- **A service outlives every release.** It runs in its own Compose project, so no
-  deploy and no rollback can stop it or remove its volume.
+- **A service outlives every release.** It runs in its own Compose project.
+  Deploy can recreate PostgreSQL services with declared extensions when their
+  configuration changes; otherwise, deploy and rollback leave services running.
+  Neither removes their volumes.
 - **Its credential is generated on the server, once.** Not in your project, not
   in the generated runtime, not in the digest. Never rotated by a re-apply.
 - **Its version binds into the release digest**, so a database upgrade under an