SELECT 
  a.category_id 
FROM 
  cscart_categories as a 
  LEFT JOIN cscart_categories as b ON b.category_id IN (166) 
WHERE 
  a.id_path LIKE CONCAT(b.id_path, '/%')

Query time 0.00038

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE b const PRIMARY,p_category_id PRIMARY 3 const 1
1 SIMPLE a ALL 83 Using where

Result

category_id
165
167
168
169
170
171
172
174
175
176
177
178
179
180
181
182
199
185
186
187
188
189
190
191
194
193
195
196
197
198
200
201
202
234
235
236
237
238
240
254
255
263