PG cannot execute UPDATE in a read-only transaction出现这种情况时，说明SQL语句可能是运行在一个PG集群中的非master节点上。查看data/pg_hba.conf。

　　SELECT pg_is_in_recovery();

   This cannot return true in a master server because the only time a master server can be in recovery。

　　还有如下可能Normally the most plausible reasons for this kind of error are :

• trying create statements on a read-only replica (the entire instance is read-only).

• <username> has default_transaction_read_only set to ON

• the database has default_transaction_read_only set to ON

  alter database exercises set default_transaction_read_only = off;

      https://stackoverflow.com/questions/31186414/error-cannot-execute-create-table-in-a-read-only-transaction

　　How to add column if not exists on PostgreSQL

　　With Postgres 9.6 this can be done using the option if not exists

　　ALTER TABLE table_name ADD COLUMN IF NOT EXISTS column_name INTEGER;

　　https://stackoverflow.com/questions/12597465/how-to-add-column-if-not-exists-on-postgresql