SELECT 
  product_id 
FROM 
  cscart_product_variation_group_products 
WHERE 
  group_id IN (13, 11)

Query time 0.00033

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_variation_group_products index idx_group_id PRIMARY 6 35 Using where; Using index

Result

product_id
1
149
286
287
288
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355