perf(psql): use EXISTS for view checks

dialect/psql/view.go

@@ -147,8 +147,11 @@ func (v *ViewQuery[T, Tslice]) Count(ctx context.Context, exec bob.Executor) (in
 
 // Exists checks if there is any matching row
 func (v *ViewQuery[T, Tslice]) Exists(ctx context.Context, exec bob.Executor) (bool, error) {
-	count, err := v.Count(ctx, exec)
-	return count > 0, err
+	ctx, err := v.RunHooks(ctx, exec)
+	if err != nil {
+		return false, err
+	}
+	return bob.One(ctx, exec, Select(sm.Columns(Exists(v.BaseQuery))), scan.SingleColumnMapper[bool])
 }
 
 // asCountQuery clones and rewrites an existing query to a count query

dialect/psql/view_test.go

@@ -3,6 +3,7 @@ package psql
 import (
 	"bytes"
 	"context"
+	"strings"
 	"testing"
 
 	_ "github.com/lib/pq"
@@ -77,6 +78,39 @@ func TestSomeViewQueryWithoutSchema(t *testing.T) {
 	}
 }
 
+func TestSomeViewExistsUsesExistsExpression(t *testing.T) {
+	ctx := context.Background()
+	if _, err := testDB.ExecContext(ctx, `CREATE TABLE exists_view_test (id BIGINT PRIMARY KEY)`); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if _, err := testDB.ExecContext(ctx, `DROP TABLE exists_view_test`); err != nil {
+			t.Error(err)
+		}
+	})
+	if _, err := testDB.ExecContext(ctx, `INSERT INTO exists_view_test (id) VALUES (1)`); err != nil {
+		t.Fatal(err)
+	}
+
+	view := NewView[int64, bob.Expression]("", "exists_view_test", Quote("id"))
+	var query strings.Builder
+	exists, err := view.Query(
+		sm.With("matching").As(Select(sm.Columns("id"), sm.From("exists_view_test"))),
+		sm.From("matching"),
+		sm.Where(Quote("id").EQ(Arg(1))),
+	).Exists(ctx, bob.DebugToWriter(testDB, &query))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !exists {
+		t.Fatal("expected matching row to exist")
+	}
+	got := query.String()
+	if !strings.Contains(got, "EXISTS ((") || !strings.Contains(got, "WITH") || strings.Contains(got, "count(1)") {
+		t.Fatalf("expected EXISTS query, got:\n%s", got)
+	}
+}
+
 func selectToString(t *testing.T, query bob.BaseQuery[*dialect.SelectQuery], argsLen int) string {
 	t.Helper()
 	ctx := context.Background()