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.
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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