Index Sort

在Elasticsearch中，索引排序（Index Sort）是一个重要的优化功能，可以在索引时对文档进行预排序。以下是详细介绍：

什么是Index Sort

Index Sort允许你在创建索引时指定一个或多个字段作为排序依据，Elasticsearch会在索引文档时按照指定的顺序存储文档。

配置Index Sort

创建带有Index Sort的索引

PUT /my_index
{
  "settings": {
    "index": {
      "sort.field": ["timestamp", "user_id"],
      "sort.order": ["desc", "asc"]
    }
  },
  "mappings": {
    "properties": {
      "timestamp": {
        "type": "date"
      },
      "user_id": {
        "type": "keyword"
      },
      "message": {
        "type": "text"
      }
    }
  }
}

多字段排序配置

PUT /sales_index
{
  "settings": {
    "index": {
      "sort.field": ["date", "amount", "region"],
      "sort.order": ["desc", "desc", "asc"]
    }
  },
  "mappings": {
    "properties": {
      "date": {"type": "date"},
      "amount": {"type": "double"},
      "region": {"type": "keyword"}
    }
  }
}

Index Sort的优势

1. 查询性能提升

GET /my_index/_search
{
  "query": {
    "range": {
      "timestamp": {
        "gte": "2024-01-01",
        "lte": "2024-01-31"
      }
    }
  },
  "sort": [
    {"timestamp": {"order": "desc"}}
  ]
}

2. 早期终止（Early Termination）

当查询结果已经按照索引排序字段排序时，Elasticsearch可以提前终止搜索：

GET /my_index/_search
{
  "query": {"match_all": {}},
  "sort": [{"timestamp": {"order": "desc"}}],
  "size": 10,
  "terminate_after": 100
}

3. 聚合性能优化

对于按排序字段进行的聚合操作，性能会显著提升：

GET /my_index/_search
{
  "size": 0,
  "aggs": {
    "daily_stats": {
      "date_histogram": {
        "field": "timestamp",
        "calendar_interval": "day"
      }
    }
  }
}

使用限制和注意事项

1. 支持的字段类型

• keyword
• numeric (long, integer, short, byte, double, float, half_float)
• date
• boolean

2. 不支持的字段类型

• text
• geo_point
• geo_shape
• 嵌套字段

3. 性能考虑

// 好的实践：使用高基数字段作为主要排序字段
{
  "sort.field": ["timestamp", "user_id"],
  "sort.order": ["desc", "asc"]
}

// 避免：使用低基数字段作为主要排序字段
{
  "sort.field": ["status", "timestamp"],  // status可能只有几个值
  "sort.order": ["asc", "desc"]
}

实际应用场景

1. 时间序列数据

PUT /logs_index
{
  "settings": {
    "index": {
      "sort.field": ["@timestamp", "severity"],
      "sort.order": ["desc", "desc"]
    }
  }
}

2. 电商数据

PUT /products_index
{
  "settings": {
    "index": {
      "sort.field": ["category", "price", "rating"],
      "sort.order": ["asc", "desc", "desc"]
    }
  }
}

3. 用户活动数据

PUT /user_activity
{
  "settings": {
    "index": {
      "sort.field": ["user_id", "activity_time"],
      "sort.order": ["asc", "desc"]
    }
  }
}

监控和验证

查看索引排序配置

GET /my_index/_settings

验证排序效果

GET /my_index/_search
{
  "query": {"match_all": {}},
  "sort": [{"timestamp": {"order": "desc"}}],
  "size": 5
}

Index Sort是Elasticsearch中一个强大的优化功能，特别适用于有明确排序需求的场景，可以显著提升查询和聚合的性能。