改进代码res := common.DB.Raw("SELECT * FROM(SELECT * FROM flaws WHERE facility_id = ? AND label LIKE ? "+ "UNION SELECT * FROM issues WHERE facility_id = ? AND label LIKE ? "+ "UNION SELECT * FROM records WHERE facility_id = ? AND label LIKE ? "+ "UNION SELECT * FROM qualities WHERE facility_id = ? AND label LIKE ? "+ "UNION SELECT * FROM inspections WHERE facility_id = ? AND label LIKE ?)"+ "AS combined_tables LIMIT ? OFFSET ?", code, "%"+strconv.Itoa(label)+"%", code, strconv.Itoa(label)+"%", code, strconv.Itoa(label)+"%", code, strconv.Itoa(label)+"%", code, strconv.Itoa(label)+"%", pageSize, offset).Find(&req)

在这段代码中，可以使用参数化查询来防止 SQL 注入攻击。另外，可以考虑将查询语句拆分成多个简单查询，然后使用 Go 语言的 goroutine 将它们并行执行，以提高查询效率。以下是修改后的代码：

res1 := common.DB.Raw("SELECT * FROM flaws WHERE facility_id = ? AND label LIKE ?", code, "%"+strconv.Itoa(label)+"%")
res2 := common.DB.Raw("SELECT * FROM issues WHERE facility_id = ? AND label LIKE ?", code, strconv.Itoa(label)+"%")
res3 := common.DB.Raw("SELECT * FROM records WHERE facility_id = ? AND label LIKE ?", code, strconv.Itoa(label)+"%")
res4 := common.DB.Raw("SELECT * FROM qualities WHERE facility_id = ? AND label LIKE ?", code, strconv.Itoa(label)+"%")
res5 := common.DB.Raw("SELECT * FROM inspections WHERE facility_id = ? AND label LIKE ?", code, strconv.Itoa(label)+"%")

var wg sync.WaitGroup
wg.Add(5)

go func() {
    defer wg.Done()
    res1.Find(&req)
}()

go func() {
    defer wg.Done()
    res2.Find(&req)
}()

go func() {
    defer wg.Done()
    res3.Find(&req)
}()

go func() {
    defer wg.Done()
    res4.Find(&req)
}()

go func() {
    defer wg.Done()
    res5.Find(&req)
}()

wg.Wait()

// Pagination
total := len(req)
if pageSize > 0 && offset >= 0 {
    res = req[offset:min(offset+pageSize, total)]
} else {
    res = req
}

注意，这里只是将查询语句拆分成多个简单查询，并行执行。如果需要进一步提高查询效率，可以考虑使用缓存来避免重复查询。