URGENET: ALTER statement exectuation hangs when it is executed multiple time on a table

I am using a shell script to copy some table records into PostgreSQL database. The logic steps are:
 
  1. set default value for table id
  2. load some large records into the table
  3. drop the id definition
 
  The actual PostgreSQL commands are:
     ALTER TABLE warngrid ALTER COLUMN id SET DEFAULT (nextval('warngrid_seq')* 10)+1;
     COPY warngrid FROM 'warngrid.csv' CSV HEADER; 
     ALTER TABLE warngrid ALTER COLUMN id DROP DEFAULT;
 
For the first time, these operation execute fine.  Then I do these steps:
  4. delete all the records in the table
  5. rerun the shell script to copy records.
 
Then, the ALTER statement in step 1 hanging there. I did check the lock state of the warngrid table, it says the process created by the ALTER statement hold an "AccessExclusiveLock" on the table. 
 
I wonder if PostgreSQL got some internal mechanism which prevent a table id field being altered many times when there are some pre-existed records in the table? or something in PostgreSQL that I am not aware of cause the problem. 
 
Thanks for the help!
 
Dennis
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