重构 PL/pgSQL 函数以返回各种 SELECT 查询的输出 [英] Refactor a PL/pgSQL function to return the output of various SELECT queries

问题描述

我编写了一个函数，它以文本形式输出格式良好的 PostgreSQL SELECT 查询.现在我不想再输出文本了，但实际上对数据库运行生成的 SELECT 语句并返回结果 - 就像查询本身一样.

I wrote a function that outputs a PostgreSQL SELECT query well formed in text form. Now I don't want to output a text anymore, but actually run the generated SELECT statement against the database and return the result - just like the query itself would.

CREATE OR REPLACE FUNCTION data_of(integer)
  RETURNS text AS
$BODY$
DECLARE
   sensors varchar(100);   -- holds list of column names
   type    varchar(100);   -- holds name of table
   result  text;           -- holds SQL query
       -- declare more variables

BEGIN
      -- do some crazy stuff

      result := 'SELECT
Datahora,' || sensors ||
      '

FROM
' || type ||
      '

WHERE
id=' || $1 ||'

ORDER BY Datahora;';

      RETURN result;
END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE;
ALTER FUNCTION data_of(integer) OWNER TO postgres;

sensors 保存表 type 的列名列表.这些是在函数的过程中声明和填充的.最终，它们的值如下:

sensors holds the list of column names for the table type. Those are declared and filled in the course of the function. Eventually, they hold values like:

• 传感器:'column1, column2, column3'
  除了 Datahora (timestamp) 所有列的类型都是 double precision.

• sensors: 'column1, column2, column3'
  Except for Datahora (timestamp) all columns are of type double precision.

type :'myTable'
可以是四个表之一的名称.每个都有不同的列，除了公共列 Datahora.

基础表的定义.

变量sensors 将保存此处显示的type 中对应表的所有 列.例如:如果 type 是 pcdmet 那么 sensors 将是 'datahora,dirvento,precipitacao,pressaoatm,radsolacum,tempar,umidrel,velvento'

The variable sensors will hold all columns displayed here for the corresponding table in type. For example: If type is pcdmet then sensors will be 'datahora,dirvento,precipitacao,pressaoatm,radsolacum,tempar,umidrel,velvento'

变量用于构建存储在result 中的SELECT 语句.喜欢:

The variables are used to build a SELECT statement that is stored in result. Like:

SELECT Datahora, column1, column2, column3
FROM   myTable
WHERE  id=20
ORDER  BY Datahora;

现在，我的函数将此语句返回为 text.我复制粘贴并在 pgAdmin 中或通过 psql 执行它.我想自动化这个，自动运行查询并返回结果.我该怎么做?

Right now, my function returns this statement as text. I copy-paste and execute it in pgAdmin or via psql. I want to automate this, run the query automatically and return the result. How can I do that?

推荐答案

动态SQL和RETURN类型

^{(我把最好的留到最后，继续阅读！)}
您要执行动态 SQL.原则上，借助 执行.您不需要 光标.事实上，大多数情况下，没有显式游标会更好.

Dynamic SQL and RETURN type

^{(I saved the best for last, keep reading!)}
You want to execute dynamic SQL. In principal, that's simple in plpgsql with the help of EXECUTE. You don't need a cursor. In fact, most of the time you are better off without explicit cursors.

您遇到的问题:您想要返回尚未定义类型的记录.函数需要在 RETURNS 子句(或带有 OUT 或 INOUT 参数).在您的情况下，您将不得不回退到匿名记录，因为返回的列的 number、names 和 types 各不相同.喜欢:

The problem you run into: you want to return records of yet undefined type. A function needs to declare its return type in the RETURNS clause (or with OUT or INOUT parameters). In your case you would have to fall back to anonymous records, because number, names and types of returned columns vary. Like:

CREATE FUNCTION data_of(integer)
  RETURNS SETOF record AS ...

但是，这并不是特别有用.您必须在每次调用时提供列定义列表.喜欢:

However, this is not particularly useful. You have to provide a column definition list with every call. Like:

SELECT * FROM data_of(17)
AS foo (colum_name1 integer
      , colum_name2 text
      , colum_name3 real);

但是，如果您事先不知道列，您会怎么做呢?
您可以使用结构较少的文档数据类型，例如 json、jsonb、hstore 或 xml.见:

But how would you even do this, when you don't know the columns beforehand?
You could use less structured document data types like json, jsonb, hstore or xml. See:

• 如何在数据库中存储数据表?

但是，就这个问题而言，我们假设您希望尽可能多地返回单独、正确键入和命名的列.

But, for the purpose of this question, let's assume you want to return individual, correctly typed and named columns as much as possible.

列 datahora 似乎是给定的，我假设数据类型 timestamp 并且总是有另外两列具有不同的名称和数据类型.

The column datahora seems to be a given, I'll assume data type timestamp and that there are always two more columns with varying name and data type.

名称我们将放弃返回类型中的通用名称.
Types 我们也将放弃，并将所有数据类型转换为 text，因为 每个 数据类型都可以转换为 text.

Names we'll abandon in favor of generic names in the return type.
Types we'll abandon, too, and cast all to text since every data type can be cast to text.

CREATE OR REPLACE FUNCTION data_of(_id integer)
  RETURNS TABLE (datahora timestamp, col2 text, col3 text)
  LANGUAGE plpgsql AS
$func$
DECLARE
   _sensors text := 'col1::text, col2::text';  -- cast each col to text
   _type    text := 'foo';
BEGIN
   RETURN QUERY EXECUTE '
      SELECT datahora, ' || _sensors || '
      FROM   ' || quote_ident(_type) || '
      WHERE  id = $1
      ORDER  BY datahora'
   USING  _id;
END
$func$;

变量_sensors和_type可以作为输入参数.

The variables _sensors and _type could be input parameters instead.

注意 RETURNS TABLE子句.

注意使用RETURN QUERY EXECUTE.这是从动态查询中返回行的更优雅的方法之一.

Note the use of RETURN QUERY EXECUTE. That is one of the more elegant ways to return rows from a dynamic query.

我为函数参数使用了一个名称，只是为了让 RETURN QUERY EXECUTE 的 USING 子句不那么混乱.SQL 字符串中的 $1 不是指函数参数，而是指使用 USING 子句传递的值.(在这个简单的例子中，两者恰好在各自的范围内都是 $1.)

I use a name for the function parameter, just to make the USING clause of RETURN QUERY EXECUTE less confusing. $1 in the SQL-string does not refer to the function parameter but to the value passed with the USING clause. (Both happen to be $1 in their respective scope in this simple example.)

注意 _sensors 的示例值:每列都转换为类型 text.

Note the example value for _sensors: each column is cast to type text.

这种代码很容易受到SQL注入的影响.我使用 quote_ident() 来防止它.将变量 _sensors 中的几个列名集中在一起会阻止 quote_ident() 的使用(这通常是个坏主意！).确保没有坏东西可以通过其他方式出现，例如通过 quote_ident() 单独运行列名.想到一个 VARIADIC 参数...

This kind of code is very vulnerable to SQL injection. I use quote_ident() to protect against it. Lumping together a couple of column names in the variable _sensors prevents the use of quote_ident() (and is typically a bad idea!). Ensure that no bad stuff can be in there some other way, for instance by individually running the column names through quote_ident() instead. A VARIADIC parameter comes to mind ...

对于 9.1 或更高版本，您可以使用 format() 进一步简化:

With version 9.1 or later you can use format() to further simplify:

RETURN QUERY EXECUTE format('
   SELECT datahora, %s  -- identifier passed as unescaped string
   FROM   %I            -- assuming the name is provided by user
   WHERE  id = $1
   ORDER  BY datahora'
  ,_sensors, _type)
USING  _id;

同样，单个列名可以正确转义，这将是一种干净的方式.

Again, individual column names could be escaped properly and would be the clean way.

在您的问题更新后，您的返回类型似乎有

After your question updates it looks like your return type has

• 可变的列数
• 但所有列的类型双精度(别名float8)

在这种情况下使用 ARRAY 类型来嵌套可变数量的值.此外，我返回一个列名数组:

Use an ARRAY type in this case to nest a variable number of values. Additionally, I return an array with column names:

CREATE OR REPLACE FUNCTION data_of(_id integer)
  RETURNS TABLE (datahora timestamp, names text[], values float8[])
  LANGUAGE plpgsql AS
$func$
DECLARE
   _sensors text := 'col1, col2, col3';  -- plain list of column names
   _type    text := 'foo';
BEGIN
   RETURN QUERY EXECUTE format('
      SELECT datahora
           , string_to_array($1)  -- AS names
           , ARRAY[%s]            -- AS values
      FROM   %s
      WHERE  id = $2
      ORDER  BY datahora'
    , _sensors, _type)
   USING  _sensors, _id;
END
$func$;

各种完整的表格类型

要真正返回表的所有列，有一个简单而强大的解决方案，使用 多态类型:

Various complete table types

To actually return all columns of a table, there is a simple, powerful solution using a polymorphic type:

CREATE OR REPLACE FUNCTION data_of(_tbl_type anyelement, _id int)
  RETURNS SETOF anyelement
  LANGUAGE plpgsql AS
$func$
BEGIN
   RETURN QUERY EXECUTE format('
      SELECT *
      FROM   %s  -- pg_typeof returns regtype, quoted automatically
      WHERE  id = $1
      ORDER  BY datahora'
    , pg_typeof(_tbl_type))
   USING  _id;
END
$func$;

打电话(重要！):

SELECT * FROM data_of(NULL::pcdmet, 17);

将调用中的 pcdmet 替换为任何其他表名.

Replace pcdmet in the call with any other table name.

anyelement 是伪数据类型，多态类型，任何非数组数据类型的占位符.函数中所有出现的 anyelement 都会评估为运行时提供的相同类型.通过将定义类型的值作为参数提供给函数，我们隐式定义了返回类型.

anyelement is a pseudo data type, a polymorphic type, a placeholder for any non-array data type. All occurrences of anyelement in the function evaluate to the same type provided at run time. By supplying a value of a defined type as argument to the function, we implicitly define the return type.

PostgreSQL 自动为每个创建的表定义一个行类型(一种复合数据类型)，因此每个表都有一个定义良好的类型.这包括临时表，方便临时使用.

PostgreSQL automatically defines a row type (a composite data type) for every table created, so there is a well defined type for every table. This includes temporary tables, which is convenient for ad-hoc use.

任何类型都可以是NULL.提交一个 NULL 值，转换为表类型:NULL::pcdmet.

Any type can be NULL. Hand in a NULL value, cast to the table type: NULL::pcdmet.

现在该函数返回一个定义明确的行类型，我们可以使用 SELECT * FROM data_of() 来分解行并获取各个列.

Now the function returns a well-defined row type and we can use SELECT * FROM data_of() to decompose the row and get individual columns.

pg_typeof(_tbl_type) 将表的名称返回为 对象标识符类型regtype.当自动转换为 text 时，如果需要，标识符会被自动双引号和模式限定，从而自动防御 SQL 注入.这甚至可以处理 quote_ident() 会失败的模式限定表名.见:

pg_typeof(_tbl_type) returns the name of the table as object identifier type regtype. When automatically converted to text, identifiers are automatically double-quoted and schema-qualified if needed, defending against SQL injection automatically. This can even deal with schema-qualified table-names where quote_ident() would fail. See:

• 表名作为 PostgreSQL 函数参数

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