What is the best way to compare two databases. Especially tables+columns, indexes+columns, constraints, views, stored procedures, triggers.
It is not about data.
It is not possible to make a network connection to the two databases, so a dblink is not a possible solution.

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select table_name from s1.user_tables
minus
select table_name from s2.user_tables

&
vice versa
&&
for all the other objects in question write sql the like

or use dbms_metadata for both schemas and then use your favourite editor to compare the files will all that ddl included
or take an exp from both schemas
imp /

@sid file=your.dmp fromuser=s1 show=yes log=s1.log
imp /

@sid file=your.dmp fromuser=s2 show=yes log=s2.log

then compare the logs and retrieve the differences

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Use OEM Change Management to take the diff between two databases. Of course you would need OMS if want to take snapshots, but for simple comparison - it's not neccessary