Unable to ALTER table after SELECT data from table

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Hi,
following I tried for some application logic based data migration

Statement stmt = dbCon.createStatement();
try {
ResultSet geo_columns_rs = stmt.
executeQuery("SELECT baseline_check_version from geodb limit 1");
source_version = Versions.R03_00;
source_version = Versions.R03_00;
}catch (Exception e) {}

to check if column already exists...
If I get an exception in Java I remember that and try to update my
database structure to new version using following:

dbCon.createStatement().execute("ALTER TABLE geodb ADD COLUMN
baseline_check_version VARCHAR(32)");

If I do so I get following error by Postgres:

org.postgresql.util.PSQLException: ERROR: current transaction is
aborted, commands ignored until end of transaction block
at
org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:1512)
at
org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1297)
at
org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:188)
at
org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:437)
at
org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:339)
at
org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:331)
at
de.airbus.omts.geodbgen.communication.dbms.Checker.execute(Checker.java:244)
at
de.airbus.omts.geodbgen.communication.dbms.Checker.migrateDB(Checker.java:206)
at
de.airbus.omts.geodbgen.communication.dbms.Checker.main(Checker.java:390)

To work around this I add an dbCon.rollBack() after select statement
above in good and in bad times. After that ALTER works.
Can someone explain me why I need this rollback ?
Using:
Postgres 8.1.4-1, JDBC postgresql-8.1-405.jdbc3, JDK 1.6
Same problem with JDK 1.5...

Thanks

thomas
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