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一 了解InfluxDB的必要性

时序数据库主要存放的数据

Time series data is a series of data points each associated with a specific time. Examples include:

• Server performance metrics
• Financial averages over time
• Sensor data, such as temperature, barometric pressure, wind speeds, etc.

时序数据库和关系数据库的区别

Relational databases can be used to store and analyze time series data, but depending on the precision of your data, a query can involve potentially millions of rows. InfluxDB is purpose-built to store and query data by time, providing out-of-the-box functionality that optionally downsamples data after a specific age and a query engine optimized for time-based data.

二 基本概念

2.1 database & duration

database

A logical container for users, retention policies, continuous queries, and time series data.

duration

The attribute of the retention policy that determines how long InfluxDB stores data. Data older than the duration are automatically dropped from the database. 

2.2 field

The key-value pair in an InfluxDB data structure that records metadata and the actual data value. Fields are required in InfluxDB data structures and they are not indexed – queries on field values scan all points that match the specified time range and, as a result, are not performant relative to tags.

Field keys are strings and they store metadata.Field values are the actual data; they can be strings, floats, integers, or booleans. A field value is always associated with a timestamp.

2.3 Tags

Tags are optional. The key-value pair in the InfluxDB data structure that records metadata.You don’t need to have tags in your data structure, but it’s generally a good idea to make use of them because, unlike fields, tags are indexed. This means that queries on tags are faster and that tags are ideal for storing commonly-queried metadata.

Tags 与  fields 的区别

Tags are indexed and fields are not indexed. This means that queries on tags are more performant than those on fields.

Tags 与  fields 的使用场景

（1）Store commonly-queried meta data in tags

（2）Store data in tags if you plan to use them with the InfluxQL GROUP BY clause

（3）Store data in fields if you plan to use them with an InfluxQL function

（4）Store numeric values as fields (tag values only support string values)

2.4 measurement 

The measurement acts as a container for tags, fields, and the time column, and the measurement name is the description of the data that are stored in the associated fields. Measurement names are strings, and, for any SQL users out there, a measurement is conceptually similar to a table.

2.5 point

In InfluxDB, a point represents a single data record, similar to a row in a SQL database table. Each point:

• has a measurement, a tag set, a field key, a field value, and a timestamp;
• is uniquely identified by its series and timestamp.

You cannot store more than one point with the same timestamp in a series. If you write a point to a series with a timestamp that matches an existing point, the field set becomes a union of the old and new field set, and any ties go to the new field set.

2.6 series

In InfluxDB, a series is a collection of points that share a measurement, tag set, and field key. A point represents a single data record that has four components: a measurement, tag set, field set, and a timestamp. A point is uniquely identified by its series and timestamp.

series key

A series key identifies a particular series by measurement, tag set, and field key.

三 查询

3.1 正则模糊查询

1.实现查询以给定字段开始的数据

select fieldName from measurementName where fieldName=~/^给定字段/

2.实现查询以给定字段结束的数据

select fieldName from measurementName where fieldName=~/给定字段$/

3.实现查询包含给定字段数据

select fieldName from measurementName where fieldName=~/给定字段/

3.2 Select 注意事项:

必须包含field key

A query requires at least one field key in the SELECT clause to return data. If the SELECT clause only includes a single tag key or several tag keys, the query returns an empty response. This behavior is a result of how the system stores data.

3.3 Where 限定

使用单引号，否则无数据返回或报错

（1）Single quote string field values in the WHERE clause. Queries with unquoted string field values or double quoted string field values will not return any data and, in most cases,will not return an error.

（2）Single quote tag values in the WHERE clause. Queries with unquoted tag values or double quoted tag values will not return any data and, in most cases, will not return an error.

3.4 Group By 

（1）Note that the GROUP BY clause must come after the WHERE clause.

（2）The GROUP BY clause groups query results by:  one or more specified tags ；specified time interval。

（3）You cannot use GROUP BY to group fields.

（4）fill() changes the value reported for time intervals that have no data.

By default, a GROUP BY time() interval with no data reports null as its value in the output column. fill() changes the value reported for time intervals that have no data. Note that fill() must go at the end of the GROUP BY clause if you’reGROUP(ing) BY several things (for example, both tags and a time interval).

3.5 ORDER BY time DESC

By default, InfluxDB returns results in ascending time order; the first point returned has the oldest timestamp and the last point returned has the most recent timestamp.ORDER BY time DESC reverses that order such that InfluxDB returns the points with the most recent timestamps first.

注意：ORDER by time DESC must appear after the GROUP BY clause if the query includes a GROUP BY clause. ORDER by time DESC must appear after the WHERE clause if the query includes a WHERE clause and no GROUP BY clause.

四.SHOW CARDINALITY

是用于估计或精确计算measurement、序列、tag key、tag value和field key的基数的一组命令。

SHOW CARDINALITY命令有两种可用的版本：估计和精确。估计值使用草图进行计算，对于所有基数大小来说，这是一个安全默认值。精确值是直接对TSM（Time-Structured Merge Tree）数据进行计数，但是，对于基数大的数据来说，运行成本很高。

下面以tag key、tag value为例。

4.1 SHOW TAG KEY CARDINALITY

估计或精确计算tag key集的基数。

ON <database>、FROM <sources>、WITH KEY = <key>、WHERE <condition>、GROUP BY <dimensions>和LIMIT/OFFSET子句是可选的。当使用这些查询子句时，查询将回退到精确计数（exect count）。当启用Time Series Index（TSI）时，才支持对time进行过滤。不支持在WHERE子句中使用time。

举例:

-- show estimated tag key cardinality
SHOW TAG KEY CARDINALITY

----计算精确值 -- show exact tag key cardinality
SHOW TAG KEY EXACT CARDINALITY

4.2 SHOW TAG VALUES CARDINALITY

估计或精确计算指定tag key对应的tag value的基数。

ON <database>、FROM <sources>、WITH KEY = <key>、WHERE <condition>、GROUP BY <dimensions>和LIMIT/OFFSET子句是可选的。当使用这些查询子句时，查询将回退到精确计数（exect count）。当启用Time Series Index（TSI）时，才支持对time进行过滤。

举例

-- show estimated tag key values cardinality for a specified tag key
SHOW TAG VALUES CARDINALITY WITH KEY = "myTagKey"

-- show estimated tag key values cardinality for a specified tag key
SHOW TAG VALUES CARDINALITY WITH KEY = "myTagKey"

-----计算精确值

-- show exact tag key values cardinality for a specified tag key
SHOW TAG VALUES EXACT CARDINALITY WITH KEY = "myTagKey"

-- show exact tag key values cardinality for a specified tag key
SHOW TAG VALUES EXACT CARDINALITY WITH KEY = "myTagKey"

4.3 应用场景举例

例如，前面的分享，我们通过Telegraf 将server的监控数据保存到了InfluxDB中，其中CPU指标是必不可少的（telegraf.conf 设置）。假如有一天，我们需要统计telegraf一共部署了多少台。其实就可以通过SHOW TAG VALUES EXACT CARDINALITY 获得。

SQL 语句如下:

SHOW TAG VALUES EXACT CARDINALITY from "cpu" WITH KEY = "host"

即查看cpu 中 host 的key值有多少个。因为通过telegraf.conf的设置，一台Server 对应一个唯一的host值，host值有多少个，就有多少台Server已部署了telegraf。

5 Drop 与 Delete

5.1 series

The DROP SERIES query deletes all points from a series in a database, and it drops the series from the index.

The query takes the following form, where you must specify either the FROM clause or the WHERE clause.

语法如下：

DROP SERIES FROM <measurement_name[,measurement_name]> WHERE <tag_key>="<tag_value>"

A successful DROP SERIES query returns an empty result.

Drop all points in the series that have a specific tag pair from all measurements in the database(即，如不指定from，将会把符合条件的所有表tag数据删除).

与Delete series  的区别是：

The DELETE query deletes all points from a series in a database. UnlikeDROP SERIES, DELETE does not drop the series from the index.

5.2 measurement_name

DELETE FROM <measurement_name> WHERE [<tag_key>="<tag_value>"] | [<time interval>]

只允许根据tag和时间来进行删除操作.

measurement的drop,是比较消耗资源的，并且操作时间相对较长。看有网友的分享，建议 在 drop measurement 之前先删除所有的 tag。

即先执行:

DROP SERIES FROM "measurement_name"

然后再执行:

drop measurement <measurement_name>

六 常用函数部分

 常用函数汇总如下:

参考网址:

https://blog.csdn.net/xuxiannian/article/details/103559246

https://blog.csdn.net/funnyPython/article/details/89888972