decimal precision constrained

docs/fixed-scale-decimal.md

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+# Decimal column policy
+
+Use the bare `numeric` column type for ordinary exact base-10 values. It stores
+the value without a schema-defined decimal-place limit and is the recommended
+choice for quantities, percentages, rates, ratios and measurements. Use
+`money` for currency amounts; its currency and rounding policy are separate
+from this document.
+
+`decimal(p,s)` is an advanced, opt-in compatibility type for strict external
+data contracts. `p` limits the total digits and `s` limits the fractional
+digits. It is hidden from the normal table-designer type list until fixed-scale
+decimal is enabled for that editing session.
+
+Fixed scale is intentionally strict:
+
+- API input outside the declared precision or scale is rejected, never
+  silently rounded by PostgreSQL.
+- A table script targeting a fixed-scale column must round explicitly before
+  returning its value.
+- A fixed-scale type cannot be changed through the table editor after the
+  table contains data.
+- Display precision, allowed increments, minimums and maximums are business
+  validation concerns; `decimal(p,s)` is not a substitute for them.
+
+Existing fixed-scale columns remain supported. The opt-in changes discovery,
+not storage compatibility.