012: cap parallelism and disable jit for shared 2 vCPU

max_parallel_workers_per_gather=1, max_parallel_workers=2, jit=off.
All reloadable - no restart.

migrations/012-tune-cpu.sh

@@ -0,0 +1,77 @@
+#!/usr/bin/env bash
+# 012-tune-cpu - Cap query parallelism for the shared-core 2 vCPU e2-micro
+# (C-tier of the 2026-08 tuning review).
+#
+#   max_parallel_workers_per_gather = 1  one query must not take every core;
+#                                        other sessions still get CPU
+#   max_parallel_workers             = 2  there are only 2 cores to begin with
+#   jit                              = off  JIT compile cost outweighs gains for
+#                                        short queries / low TPS / shared CPU
+#
+# All three are reloadable GUCs - no restart, no downtime.
+# up:    write drop-in; reload
+# down:  remove drop-in; reload
+set -euo pipefail
+
+PGBIN=/etc/postgresql/18/main
+CONF=$PGBIN/conf.d/012-tune-cpu.conf
+CLUSTER=$(pg_lsclusters --no-header | awk 'NR==1{print $1"/"$2}')
+
+# desired state as "guc value" pairs; SHOW renders exactly these strings
+WANT=(
+  'max_parallel_workers_per_gather 1'
+  'max_parallel_workers 2'
+  'jit off'
+)
+
+pg() { runuser -u postgres -- psql -d postgres -v ON_ERROR_STOP=1 -tAc "$1"; }
+
+write_conf() {
+  install -d -m 755 "$PGBIN/conf.d"
+  cat > "$CONF" <<'EOF'
+# managed by sysmig 012-tune-cpu - parallelism caps for shared-core 2 vCPU
+max_parallel_workers_per_gather = 1
+max_parallel_workers = 2
+jit = off
+EOF
+  chown root:postgres "$CONF"
+  chmod 644 "$CONF"
+}
+
+case "${1:-}" in
+  up)
+    write_conf
+    pg_ctlcluster "$CLUSTER" reload
+    echo "    cluster reloaded"
+    for kv in "${WANT[@]}"; do
+      read -r guc want <<<"$kv"
+      got=$(pg "SHOW $guc")
+      [[ $got == "$want" ]] || { echo "FATAL: $guc is '$got', expected '$want'" >&2; exit 1; }
+    done
+    echo "    ${#WANT[@]} cpu/parallel settings active"
+    ;;
+  status)
+    if [[ $EUID -eq 0 ]]; then
+      q() { runuser -u postgres -- psql -d postgres -v ON_ERROR_STOP=1 -tAc "$1"; }
+    elif psql -d postgres -tAc 'SELECT 1' >/dev/null 2>&1; then
+      q() { psql -d postgres -v ON_ERROR_STOP=1 -tAc "$1"; }
+    else
+      echo "? needs root or peer DB access to verify"
+      exit 2
+    fi
+    [[ -f $CONF ]] || { echo "DRIFT drop-in $CONF missing"; exit 1; }
+    bad=()
+    for kv in "${WANT[@]}"; do
+      read -r guc want <<<"$kv"
+      got=$(q "SHOW $guc" 2>/dev/null)
+      [[ $got == "$want" ]] || bad+=("$guc=$got(want $want)")
+    done
+    if ((${#bad[@]})); then echo "DRIFT ${bad[*]}"; exit 1; fi
+    echo "OK parallel capped (per_gather=1, workers=2), jit off"
+    ;;
+  down)
+    rm -f "$CONF"
+    pg_ctlcluster "$CLUSTER" reload
+    echo "    drop-in removed, cluster reloaded (defaults restored)"
+    ;;
+esac