Postgresql updating millions of rows
ROLLBACK and COMMIT have the same semantic meaning in both systems; also SAVEPOINTS mean the same.
Postgres knows all the isolation levels Oracle knows (and a few more).
If expr is equal to a search, Oracle returns the corresponding result. This also applies within Oracle, and actually, anybody using DECODE in times of Oracle 12c should roast in hell. NVL returns its first argument if it is not null, otherwise it returns its second argument.
If no match is found, Oracle returns default, or, if default is omitted, returns null. The above statement will return SYSDATE if hire_date is null.
On conversion Oracle to Postgre SQL You will need to carefully analyze all your string handling code and separate NULLs and empty strings. The appropriate data type in Postgres is DECIMAL or NUMERIC, both are equivalent.
The limit for numbers in Postgres (up to 131072 digits before the decimal point; up to 16383 digits after the decimal point) is much higher than in Oracle and is internally stored in a similar way.