PostgreSQL's read only transaction mode

When a transaction is read-only, the following SQL commands are disallowed: INSERT,UPDATE, DELETE, and COPY FROM if the table they would write to is not a temporary table; allCREATE, ALTER, and DROP commands; COMMENT, GRANT, REVOKE, TRUNCATE; and EXPLAIN ANALYZE and EXECUTE if the command they would execute is among those listed. This is a high-level notion of read-only that does not prevent all writes to disk. 

mydb=# begin;
BEGIN
mydb=#  set transaction read only;                           
SET

--插入数据失败
mydb=# insert into t values(generate_series(1,10000),'rudy');
ERROR:  cannot execute INSERT in a read-only transaction