在架构中的列条目上选择表（有很多表）

Question

• 我在 PostgreSQL-DB 中有一个架构，其中包含大约。3000 个表，每个表都有一行/条目（大小、id、geom）。我想选择大小值小于 20 的所有表。如何查询多个表而不加入它们？

• 将所有这些表合并为一个并执行一个简单的 SELECT 查询会更好吗？

Answer 1

如果您在合并所有数据时需要帮助，您可以这样做：

DO $DO$
DECLARE
  _tab text;
  _r record;
BEGIN
  FOR _r IN
    -- refine the following query according to your particular needs, if the following doesn't work well
    SELECT a.attrelid::regclass::text AS table_name
    FROM pg_attribute a
    JOIN pg_class c ON (c.oid = a.attrelid)
    WHERE a.attrelid IN (SELECT attrelid FROM pg_attribute WHERE attname ILIKE 'geom') AND -- any table with a "geom" column name
          c.relnamespace > 11 AND c.relkind = 'r'
    GROUP BY a.attrelid
    HAVING COUNT(*) = 3 -- only tables with three columns
    ORDER BY 1
   LOOP
     IF _tab IS NULL THEN
       _tab := _r.table_name;
       EXECUTE 'CREATE TEMP TABLE merged_multi_polygons (tablename text, LIKE '||_tab||');';
     END IF;
     EXECUTE 'INSERT INTO merged_multi_polygons SELECT '||quote_literal(_r.table_name)||', * FROM '||_r.table_name
             -- ||' WHERE size < 20' -- UNCOMMENT THIS LINE TO PRODUCE THE TABLE ONLY FOR SMALL-SIZED ENTRIES
             || ' LIMIT 1';
   END LOOP;
END;
$DO$;

SELECT * FROM merged_multi_polygons WHERE size < 20;