diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 89365611a0e..1696540445f 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -92,7 +92,7 @@ jobs:
 
       - name: Run IO tests
         if: always()
-        run: postgrest-with-postgresql-${{ matrix.pgVersion }} -f test/io/fixtures.sql postgrest-test-io -vv
+        run: postgrest-with-postgresql-${{ matrix.pgVersion }} postgrest-test-io -vv
 
 
   Test-Memory-Nix:
diff --git a/nix/tools/loadtest.nix b/nix/tools/loadtest.nix
index 36635654b6f..6c2988b3369 100644
--- a/nix/tools/loadtest.nix
+++ b/nix/tools/loadtest.nix
@@ -60,7 +60,7 @@ let
         abs_output="$(realpath "$_arg_output")"
 
         # shellcheck disable=SC2145
-        ${withTools.withPg} --fixtures "$_arg_testdir"/fixtures.sql \
+        ${withTools.withPg} -f "$_arg_testdir"/fixtures.sql \
         ${withTools.withSlowPg} \
         ${withTools.withPgrst} \
         ${withTools.withSlowPgrst} \
diff --git a/nix/tools/memory.nix b/nix/tools/memory.nix
index bdfa3c5da3a..f147eaabe33 100644
--- a/nix/tools/memory.nix
+++ b/nix/tools/memory.nix
@@ -17,7 +17,7 @@ let
         withPath = [ postgrestProfiled curl ];
       }
       ''
-        ${withTools.withPg} test/memory/memory-tests.sh
+        ${withTools.withPg} -f test/spec/fixtures.sql test/memory/memory-tests.sh
       '';
 
 in
diff --git a/nix/tools/tests.nix b/nix/tools/tests.nix
index 469208e9f63..d661b398a58 100644
--- a/nix/tools/tests.nix
+++ b/nix/tools/tests.nix
@@ -28,8 +28,8 @@ let
         withEnv = postgrest.env;
       }
       ''
-        ${withTools.withPg} ${cabal-install}/bin/cabal v2-run ${devCabalOptions} \
-          test:spec -- "''${_arg_leftovers[@]}"
+        ${withTools.withPg} -f test/spec/fixtures/load.sql \
+          ${cabal-install}/bin/cabal v2-run ${devCabalOptions} test:spec -- "''${_arg_leftovers[@]}"
       '';
 
   testDoctests =
@@ -59,9 +59,10 @@ let
         withEnv = postgrest.env;
       }
       ''
-        ${withTools.withPg} ${runtimeShell} -c " \
-          ${cabal-install}/bin/cabal v2-run ${devCabalOptions} test:spec && \
-          ${cabal-install}/bin/cabal v2-run ${devCabalOptions} test:spec"
+        ${withTools.withPg} -f test/spec/fixtures/load.sql \
+          ${runtimeShell} -c " \
+            ${cabal-install}/bin/cabal v2-run ${devCabalOptions} test:spec && \
+            ${cabal-install}/bin/cabal v2-run ${devCabalOptions} test:spec"
       '';
 
   ioTestPython =
@@ -99,7 +100,7 @@ let
         withPath = [ jq ];
       }
       ''
-        ${withTools.withPg} \
+        ${withTools.withPg} -f test/spec/fixtures.sql \
             ${cabal-install}/bin/cabal v2-run ${devCabalOptions} --verbose=0 -- \
             postgrest --dump-schema \
             | ${yq}/bin/yq -y .
@@ -137,11 +138,12 @@ let
 
           # collect all tests
           HPCTIXFILE="$tmpdir"/io.tix \
-            ${withTools.withPg} -f test/io/fixtures.sql ${cabal-install}/bin/cabal v2-exec ${devCabalOptions} -- \
-            ${ioTestPython}/bin/pytest -v test/io
+            ${withTools.withPg} -f test/io/fixtures.sql \
+            ${cabal-install}/bin/cabal v2-exec ${devCabalOptions} -- ${ioTestPython}/bin/pytest -v test/io
 
           HPCTIXFILE="$tmpdir"/spec.tix \
-            ${withTools.withPg} ${cabal-install}/bin/cabal v2-run ${devCabalOptions} test:spec
+            ${withTools.withPg} -f test/spec/fixtures/load.sql \
+            ${cabal-install}/bin/cabal v2-run ${devCabalOptions} test:spec
 
           # Note: No coverage for doctests, as doctests leverage GHCi and GHCi does not support hpc
 
diff --git a/nix/tools/withTools.nix b/nix/tools/withTools.nix
index 79c9802cace..51764a064b3 100644
--- a/nix/tools/withTools.nix
+++ b/nix/tools/withTools.nix
@@ -17,15 +17,14 @@ let
     { name, postgresql }:
     let
       commandName = "postgrest-with-${name}";
-      superuserRole = "postgres";
     in
     checkedShellScript
       {
         name = commandName;
-        docs = "Run the given command in a temporary database with ${name}. If you wish to mutate the database, login with the '${superuserRole}' role.";
+        docs = "Run the given command in a temporary database with ${name}. If you wish to mutate the database, login with the postgres role.";
         args =
           [
-            "ARG_OPTIONAL_SINGLE([fixtures], [f], [SQL file to load fixtures from], [test/spec/fixtures/load.sql])"
+            "ARG_OPTIONAL_SINGLE([fixtures], [f], [SQL file to load fixtures from])"
             "ARG_POSITIONAL_SINGLE([command], [Command to run])"
             "ARG_LEFTOVERS([command arguments])"
             "ARG_USE_ENV([PGUSER], [postgrest_test_authenticator], [Authenticator PG role])"
@@ -41,63 +40,64 @@ let
         withTmpDir = true;
       }
       ''
-        # avoid starting multiple layers of withTmpDb
-        if test -v PGHOST; then
-          exec "$_arg_command" "''${_arg_leftovers[@]}"
-        fi
-
         setuplog="$tmpdir/setup.log"
 
         log () {
           echo "$1" >> "$setuplog"
         }
 
-        mkdir -p "$tmpdir"/{db,socket}
-        # remove data dir, even if we keep tmpdir - no need to upload it to artifacts
-        trap 'rm -rf $tmpdir/db' EXIT
-
-        export PGDATA="$tmpdir/db"
-        export PGHOST="$tmpdir/socket"
-        export PGUSER
-        export PGDATABASE
-        export PGRST_DB_SCHEMAS
-        export PGTZ
-        export PGOPTIONS
-
-        HBA_FILE="$tmpdir/pg_hba.conf"
-        echo "local $PGDATABASE some_protected_user password" > "$HBA_FILE"
-        echo "local $PGDATABASE all trust" >> "$HBA_FILE"
-
-        log "Initializing database cluster..."
-        # We try to make the database cluster as independent as possible from the host
-        # by specifying the timezone, locale and encoding.
-        # initdb -U creates a superuser(man initdb)
-        TZ=$PGTZ initdb --no-locale --encoding=UTF8 --nosync -U "${superuserRole}" --auth=trust \
-          >> "$setuplog"
-
-        log "Starting the database cluster..."
-        # Instead of listening on a local port, we will listen on a unix domain socket.
-        pg_ctl -l "$tmpdir/db.log" -w start -o "-F -c listen_addresses=\"\" -c hba_file=$HBA_FILE -k $PGHOST -c log_statement=\"all\" " \
-          >> "$setuplog"
-
-        # shellcheck disable=SC2317
-        stop () {
-          log "Stopping the database cluster..."
-          pg_ctl stop -m i >> "$setuplog"
-          rm -rf "$tmpdir/db"
-        }
-        trap stop EXIT
-
-        log "Creating a minimally privileged $PGUSER connection role..."
-        createuser "$PGUSER" -U "${superuserRole}" --host="$tmpdir/socket" --no-createdb --no-inherit --no-superuser --no-createrole --no-replication --login
+        # Avoid starting multiple layers of withTmpDb, but make sure to have the last invocation
+        # load fixtures. Otherwise postgrest-with-postgresql-xx postgrest-test-io would not be possible.
+        if ! test -v PGHOST; then
+
+          mkdir -p "$tmpdir"/{db,socket}
+          # remove data dir, even if we keep tmpdir - no need to upload it to artifacts
+          trap 'rm -rf $tmpdir/db' EXIT
+
+          export PGDATA="$tmpdir/db"
+          export PGHOST="$tmpdir/socket"
+          export PGUSER
+          export PGDATABASE
+          export PGRST_DB_SCHEMAS
+          export PGTZ
+          export PGOPTIONS
+
+          HBA_FILE="$tmpdir/pg_hba.conf"
+          echo "local $PGDATABASE some_protected_user password" > "$HBA_FILE"
+          echo "local $PGDATABASE all trust" >> "$HBA_FILE"
+
+          log "Initializing database cluster..."
+          # We try to make the database cluster as independent as possible from the host
+          # by specifying the timezone, locale and encoding.
+          # initdb -U creates a superuser(man initdb)
+          TZ=$PGTZ initdb --no-locale --encoding=UTF8 --nosync -U postgres --auth=trust \
+            >> "$setuplog"
+
+          log "Starting the database cluster..."
+          # Instead of listening on a local port, we will listen on a unix domain socket.
+          pg_ctl -l "$tmpdir/db.log" -w start -o "-F -c listen_addresses=\"\" -c hba_file=$HBA_FILE -k $PGHOST -c log_statement=\"all\" " \
+            >> "$setuplog"
+
+          # shellcheck disable=SC2317
+          stop () {
+            log "Stopping the database cluster..."
+            pg_ctl stop -m i >> "$setuplog"
+            rm -rf "$tmpdir/db"
+          }
+          trap stop EXIT
 
-        log "Loading fixtures under the ${superuserRole} role..."
-        psql -U "${superuserRole}" -v PGUSER="$PGUSER" -v ON_ERROR_STOP=1 -f "$_arg_fixtures" >> "$setuplog"
+          log "Creating a minimally privileged $PGUSER connection role..."
+          createuser "$PGUSER" -U postgres --host="$tmpdir/socket" --no-createdb --no-inherit --no-superuser --no-createrole --no-replication --login
 
-        log "Done. Running command..."
+          echo "${commandName}: You can connect with: psql 'postgres:///$PGDATABASE?host=$tmpdir/socket' -U postgres"
+          echo "${commandName}: You can tail the logs with: tail -f $tmpdir/db.log"
+        fi
 
-        echo "${commandName}: You can connect with: psql 'postgres:///$PGDATABASE?host=$tmpdir/socket' -U ${superuserRole}"
-        echo "${commandName}: You can tail the logs with: tail -f $tmpdir/db.log"
+        if test "$_arg_fixtures"; then
+          log "Loading fixtures under the postgres role..."
+          psql -U postgres -v PGUSER="$PGUSER" -v ON_ERROR_STOP=1 -f "$_arg_fixtures" >> "$setuplog"
+          log "Done. Running command..."
+        fi
 
         ("$_arg_command" "''${_arg_leftovers[@]}")
       '';