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
valuefor 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.
rerunthe 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
processcreated 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
aresome 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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