以下是一些可以在PostgreSQL的information_schema中執行的SQL查詢：

1. 查看所有數據庫：

SELECT datname FROM information_schema.schemata;

2. 查看所有表格：

SELECT table_name FROM information_schema.tables WHERE table_schema = 'public';

3. 查看表格的列信息：

SELECT column_name, data_type, is_nullable FROM information_schema.columns WHERE table_name = 'your_table_name';

4. 查看主鍵信息：

SELECT tc.table_name, kcu.column_name
FROM information_schema.table_constraints AS tc
JOIN information_schema.key_column_usage AS kcu
ON tc.constraint_name = kcu.constraint_name
WHERE constraint_type = 'PRIMARY KEY';

5. 查看外鍵信息：

SELECT tc.table_name, kcu.column_name, ccu.table_name AS foreign_table_name, ccu.column_name AS foreign_column_name
FROM information_schema.table_constraints AS tc
JOIN information_schema.key_column_usage AS kcu
ON tc.constraint_name = kcu.constraint_name
JOIN information_schema.constraint_column_usage AS ccu
ON ccu.constraint_name = tc.constraint_name
WHERE constraint_type = 'FOREIGN KEY';

6. 查看索引信息：

SELECT indexname, indexdef FROM pg_indexes WHERE tablename = 'your_table_name';

7. 查看視圖信息：

SELECT table_name FROM information_schema.views;

8. 查看函數信息：

SELECT routine_name FROM information_schema.routines WHERE routine_type = 'FUNCTION';

這些是一些常見的SQL查詢，可以幫助您了解數據庫中的元數據信息。您還可以根據需要編寫自定義的查詢來獲取更詳細的信息。