在PostgreSQL中选择不带函数的动态列

问题描述

我需要从两个或更多表("A"，"B")中选择行.它们具有差异列，我不使用继承.

I need to select rows from two and more tables ("A", "B"). They have differences columns and I don't use inheritance for it.

所以.例如:

SELECT * FROM "A" UNION SELECT * FROM "B" 

ERROR: each UNION query must have the same number of columns

我能理解为什么.

我尝试从根表的根架构中获取相交的列:

I try get intersected columns from root schema in root table:

SELECT column_name FROM information_schema.columns
WHERE table_schema = 'client_root' AND table_name ='conditions'

没关系！但我不使用查询:

It's ok! But I don't use query:

SELECT
   (SELECT column_name FROM information_schema.columns
    WHERE table_schema = 'client_root' AND table_name ='conditions')
FROM "client_123"."A"

所以.如何将子选择数据放入根选择中?

So. How I can put sub select data in root select?

推荐答案

您试图做的事情几乎不可能全部实现.

What you are trying to do is hardly possible in its entirety.

首先，这是您可以做的所要做的:一个为此类查询创建SQL的plpgsql函数:

First, here is what you can do: a plpgsql function that creates the SQL for such a query:

CREATE OR REPLACE FUNCTION f_union_common_col_sql(text, text)
 RETURNS text
AS $function$
DECLARE 
  _cols text;
BEGIN

_cols := string_agg(attname, ', ')
FROM (
    SELECT a.attname
    FROM   pg_attribute a
    WHERE  a.attrelid = $1::regclass::oid
    AND    a.attnum >= 1
    INTERSECT
    SELECT a.attname
    FROM   pg_attribute a
    WHERE  a.attrelid = $2::regclass::oid
    AND    a.attnum >= 1
    ) x;

RETURN 'SELECT ' || _cols || '
FROM   ' || quote_ident($1) || '
UNION
SELECT ' || _cols || '
FROM   ' || quote_ident($1);

END;
$function$  LANGUAGE plpgsql;

COMMENT ON FUNCTION f_union_common_col_sql(text, text) IS 'Create SQL to query all visible columns that two tables have in common.
# Without duplicates. Use UNION ALL if you want to include duplicates.
# Depends on visibility dicatated by search_path
$1 .. table1: optionally schema-qualified, case sensitive!
$2 .. table2: optionally schema-qualified, case sensitive!';

致电:

SELECT f_union_common_col_sql('myschema1.tbl1', 'myschema2.tbl2');

为您提供完整的查询.在第二个调用中执行它.

Gives you the complete query. Execute it in a second call.

您可以在 plpgsql函数手册中找到我在此处使用的大多数内容.
PostgreSQL 9.0引入了集合函数string_agg() .在旧版本中，您将:array_to_string(array_agg(attname), ', ').

You can find most everything I used here in the manual on plpgsql functions.
The aggregate function string_agg() was introduced with PostgreSQL 9.0. In older versions you would: array_to_string(array_agg(attname), ', ').

接下来，这是您几乎无法做到的:

CREATE OR REPLACE FUNCTION f_union_common_col(text, text)
  RETURNS SETOF record AS
$BODY$
DECLARE 
  _cols text;
BEGIN

_cols := string_agg(attname, ', ')
FROM (
    SELECT a.attname
    FROM   pg_attribute a
    WHERE  a.attrelid = $1::regclass::oid
    AND    a.attnum >= 1
    INTERSECT
    SELECT a.attname
    FROM   pg_attribute a
    WHERE  a.attrelid = $2::regclass::oid
    AND    a.attnum >= 1
    ) x;

RETURN QUERY EXECUTE '
SELECT ' || _cols || '
FROM quote_ident($1)
UNION
SELECT ' || _cols || '
FROM quote_ident($2)';

END;
$BODY$
  LANGUAGE plpgsql VOLATILE;

COMMENT ON FUNCTION f_union_common_col(text, text) IS 'Query all visible columns that two tables have in common.
# Without duplicates. Use UNION ALL if you want to include duplicates.
# Depends on visibility dicatated by search_path
# !BUT! you need to specify a column definition list for every call. So, hardly useful.
$1 .. table1 (optionally schema-qualified)
$2 .. table1 (optionally schema-qualified)';

函数调用要求您指定目标列的列表.因此，这一点都没有用:

A function call requires you to specify the list of target columns. so this is hardly useful at all:

SELECT * from f_union_common_col('myschema1.tbl1', 'myschema2.tbl2')

ERROR:  a column definition list is required for functions returning "record"

没有简单的方法可以解决此问题.您将必须动态创建一个函数或至少一个复杂类型.这是我停下来的地方.

There is no easy way around this. You would have to dynamically create a function or at least a complex type. This is where I stop.

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