SELECT 
  b.row_position 
FROM 
  (
    SELECT 
      post_id, 
      @r := @r + 1 as row_position 
    FROM 
      (
        SELECT 
          cscart_cp_blog_posts.post_id 
        FROM 
          cscart_cp_blog_posts 
          INNER JOIN cscart_cp_blog_post_categories ON cscart_cp_blog_post_categories.post_id = cscart_cp_blog_posts.post_id 
          INNER JOIN cscart_cp_blog_categories ON cscart_cp_blog_categories.category_id = cscart_cp_blog_post_categories.category_id 
          AND (
            cscart_cp_blog_categories.usergroup_ids = '' 
            OR FIND_IN_SET(
              0, cscart_cp_blog_categories.usergroup_ids
            ) 
            OR FIND_IN_SET(
              1, cscart_cp_blog_categories.usergroup_ids
            )
          ) 
          AND cscart_cp_blog_categories.status IN ('A', 'H') 
          LEFT JOIN cscart_cp_blog_post_descriptions ON cscart_cp_blog_posts.post_id = cscart_cp_blog_post_descriptions.post_id 
          AND cscart_cp_blog_post_descriptions.lang_code = 'ru' 
          LEFT JOIN cscart_cp_blog_authors ON cscart_cp_blog_authors.author_id = cscart_cp_blog_posts.author_id 
        WHERE 
          1 
          AND cscart_cp_blog_posts.status IN ('A') 
          AND cscart_cp_blog_categories.company_id = 1 
          AND (
            cscart_cp_blog_categories.usergroup_ids = '' 
            OR FIND_IN_SET(
              0, cscart_cp_blog_categories.usergroup_ids
            ) 
            OR FIND_IN_SET(
              1, cscart_cp_blog_categories.usergroup_ids
            )
          ) 
          AND (
            cscart_cp_blog_posts.usergroup_ids = '' 
            OR FIND_IN_SET(
              0, cscart_cp_blog_posts.usergroup_ids
            ) 
            OR FIND_IN_SET(
              1, cscart_cp_blog_posts.usergroup_ids
            )
          ) 
          AND (
            cscart_cp_blog_posts.use_avail_period = 'N' 
            OR (
              cscart_cp_blog_posts.use_avail_period = 'Y' 
              AND cscart_cp_blog_posts.avail_from_timestamp <= 1771127366 
              AND cscart_cp_blog_posts.avail_till_timestamp >= 1771127366
            )
          ) 
          AND cscart_cp_blog_post_categories.category_id IN (7) 
        GROUP BY 
          cscart_cp_blog_posts.post_id 
        ORDER BY 
          cscart_cp_blog_posts.timestamp desc, 
          cscart_cp_blog_posts.post_id desc
      ) AS a
  ) AS b 
WHERE 
  b.post_id = 17

Query time 0.00056

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "<derived2>",
      "access_type": "ALL",
      "rows": 17,
      "filtered": 100,
      "attached_condition": "b.post_id = 17",
      "materialized": {
        "query_block": {
          "select_id": 2,
          "table": {
            "table_name": "<derived3>",
            "access_type": "ALL",
            "rows": 17,
            "filtered": 100,
            "materialized": {
              "query_block": {
                "select_id": 3,
                "filesort": {
                  "sort_key": "cscart_cp_blog_posts.`timestamp` desc, cscart_cp_blog_posts.post_id desc",
                  "temporary_table": {
                    "table": {
                      "table_name": "cscart_cp_blog_authors",
                      "access_type": "system",
                      "possible_keys": ["PRIMARY"],
                      "rows": 0,
                      "filtered": 0,
                      "const_row_not_found": true
                    },
                    "table": {
                      "table_name": "cscart_cp_blog_categories",
                      "access_type": "const",
                      "possible_keys": ["PRIMARY"],
                      "key": "PRIMARY",
                      "key_length": "3",
                      "used_key_parts": ["category_id"],
                      "ref": ["const"],
                      "rows": 1,
                      "filtered": 100
                    },
                    "table": {
                      "table_name": "cscart_cp_blog_posts",
                      "access_type": "ALL",
                      "possible_keys": ["PRIMARY", "status"],
                      "rows": 18,
                      "filtered": 94.44444275,
                      "attached_condition": "cscart_cp_blog_posts.`status` = 'A' and (cscart_cp_blog_posts.usergroup_ids = '' or find_in_set(0,cscart_cp_blog_posts.usergroup_ids) or find_in_set(1,cscart_cp_blog_posts.usergroup_ids)) and (cscart_cp_blog_posts.use_avail_period = 'N' or cscart_cp_blog_posts.use_avail_period = 'Y' and cscart_cp_blog_posts.avail_from_timestamp <= 1771127366 and cscart_cp_blog_posts.avail_till_timestamp >= 1771127366)"
                    },
                    "table": {
                      "table_name": "cscart_cp_blog_post_categories",
                      "access_type": "eq_ref",
                      "possible_keys": ["PRIMARY"],
                      "key": "PRIMARY",
                      "key_length": "6",
                      "used_key_parts": ["post_id", "category_id"],
                      "ref": [
                        "noframes_kz_new.cscart_cp_blog_posts.post_id",
                        "const"
                      ],
                      "rows": 1,
                      "filtered": 100,
                      "using_index": true
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}