也许只是重启postgres => sudo服务postgresql重启

没有必要放弃。只需删除并重新创建公共模式。在大多数情况下，这有完全相同的效果。

namespace :db do

desc 'Clear the database'
task :clear_db => :environment do |t,args|
  ActiveRecord::Base.establish_connection
  ActiveRecord::Base.connection.tables.each do |table|
    next if table == 'schema_migrations'
    ActiveRecord::Base.connection.execute("TRUNCATE #{table}")
  end
end

desc 'Delete all tables (but not the database)'
task :drop_schema => :environment do |t,args|
  ActiveRecord::Base.establish_connection
  ActiveRecord::Base.connection.execute("DROP SCHEMA public CASCADE")
  ActiveRecord::Base.connection.execute("CREATE SCHEMA public")
  ActiveRecord::Base.connection.execute("GRANT ALL ON SCHEMA public TO postgres")
  ActiveRecord::Base.connection.execute("GRANT ALL ON SCHEMA public TO public")
  ActiveRecord::Base.connection.execute("COMMENT ON SCHEMA public IS 'standard public schema'")
end

desc 'Recreate the database and seed'
task :redo_db => :environment do |t,args|
  # Executes the dependencies, but only once
  Rake::Task["db:drop_schema"].invoke
  Rake::Task["db:migrate"].invoke
  Rake::Task["db:migrate:status"].invoke 
  Rake::Task["db:structure:dump"].invoke
  Rake::Task["db:seed"].invoke
end

end

MacOS，如果postgresql安装了brew:

brew services restart postgresql

删除postgresql会话/连接

可以使用pg_terminate_backend()终止连接。你必须是超级用户才能使用这个功能。这在所有操作系统上都是一样的。

SELECT 
    pg_terminate_backend(pid) 
FROM 
    pg_stat_activity 
WHERE 
    -- don't kill my own connection!
    pid <> pg_backend_pid()
    -- don't kill the connections to other databases
    AND datname = 'database_name'
    ;

在执行这个查询之前，你必须撤销CONNECT权限以避免新的连接:

REVOKE CONNECT ON DATABASE dbname FROM PUBLIC, username;

如果你正在使用Postgres 8.4-9.1，请使用procpid而不是pid

SELECT 
    pg_terminate_backend(procpid) 
FROM 
    pg_stat_activity 
WHERE 
    -- don't kill my own connection!
    procpid <> pg_backend_pid()
    -- don't kill the connections to other databases
    AND datname = 'database_name'
    ;

案例: 查询执行失败:

DROP TABLE dbo.t_tabelname

解决方案: a.显示查询状态活动如下:

SELECT * FROM pg_stat_activity  ;

b.查找“查询”列包含的行:

'DROP TABLE dbo.t_tabelname'

c.在同一行中，获取“PID”列的值

example : 16409

d.执行以下脚本:

SELECT 
    pg_terminate_backend(25263) 
FROM 
    pg_stat_activity 
WHERE 
    -- don't kill my own connection!
    25263 <> pg_backend_pid()
    -- don't kill the connections to other databases
    AND datname = 'database_name'
    ;

SELECT 
pg_terminate_backend(pid) 
FROM 
pg_stat_activity 
WHERE
pid <> pg_backend_pid()
-- no need to kill connections to other databases
AND datname = current_database();
-- use current_database by opening right query tool