The difference that compiles and returns another value.

Every Oracle-to-PostgreSQL difference is either loud or silent, and they cost three orders of magnitude apart. Loud means the statement will not compile — NVL, DECODE, ROWNUM, MINUS, TRUNC on a date — and you find it on the first afternoon, from a stack trace, an hour each. Silent means it compiles, runs, and returns a different value, and you find it from a customer, in a quarter, in a reconciliation that will not balance. 7 of the 13 differences this page checks are silent.

The engine is free and needs no account, and it reads no database: every figure is arithmetic on the sheet you paste. Oracle has no empty string — '' IS NULL — so a || b ignores it, = '' never matches a row, and a NOT NULL VARCHAR2 enforced non-emptiness for free. PostgreSQL keeps them apart and all three stop being true: for a chain of 3 column operands, 19 of the 27 combinations return a different value, which is 3ⁿ − 2ⁿ. VARCHAR2(40) counts BYTES and varchar(40) counts CHARACTERS — 40 characters of ASCII, 13 of Chinese, 10 of emoji against 40 of any of them, up to 160 bytes, 4× the old limit — and nothing errors, because a limit getting looser is not an error. The target type decides the arithmetic: NUMBER(10) → integer makes 10 / 4 return 2 where Oracle returned 2.5, a NUMBER → double precision is exact to 9,007,199,254,740,992 and then starts skipping, and a DATE → date sets every timestamp to midnight. And Oracle’s default sort is binary, so Apricot, Banana, Zebra… becomes Ångström, apple, Apricot… under a locale collation — first difference at position 1, inside a page of 10, so a paginated query returns different rows. Every lane ships a worked example that costs nothing. Writing the report needs credits.