DISTINCT ON & string types

Latest row using DISTINCT ON:

SELECT DISTINCT ON (customer_id)
       customer_id, order_id, order_ts, amount
FROM orders
ORDER BY customer_id, order_ts DESC NULLS LAST, order_id DESC;

The leading ordering expression matches DISTINCT ON; the remaining expressions choose the surviving row. Use the window version from section 9 when portability matters.

String types:

Type	Main distinction
`TEXT`	Variable-length text without a declared length bound
`VARCHAR`	Variable-length text without a declared length bound
`VARCHAR(n)`	Variable-length text with a declared maximum length
`CHAR(n)`	Fixed-width, blank-padded character data

Do not choose CHAR(n) expecting it to be faster in PostgreSQL. Choose a length bound when it expresses a real data constraint.

References