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skills/reviewing-oracle-to-postgres-migration/references/oracle-rownum-pagination.md

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+# Oracle to PostgreSQL: ROWNUM Pagination vs LIMIT/OFFSET
+
+## Problem
+
+Oracle uses `ROWNUM` pseudo-column for pagination and row-limiting. PostgreSQL uses standard `LIMIT` / `OFFSET` syntax. `ROWNUM` is also fundamentally different in *when* it is assigned, which affects filtering behavior.
+
+## Behavior Comparison
+
+**Oracle:**
+- `ROWNUM` is assigned before `ORDER BY` — filtering with `ROWNUM` on an unordered result set is non-deterministic
+- Common pattern to get "top N ordered rows" requires a subquery:
+  ```sql
+  SELECT * FROM (SELECT * FROM t ORDER BY col) WHERE ROWNUM <= 10
+  ```
+- `ROWNUM BETWEEN n AND m` requires a double-wrapped subquery
+
+**PostgreSQL:**
+- `LIMIT n` restricts result rows after `ORDER BY` is applied — straightforward and deterministic
+- `OFFSET n` skips rows; combine with `LIMIT` for pagination
+- No `ROWNUM` pseudo-column exists
+
+## Code Example
+
+```sql
+-- Oracle: top 10 rows by date
+SELECT * FROM (
+  SELECT * FROM orders ORDER BY created_at DESC
+) WHERE ROWNUM <= 10;
+
+-- PostgreSQL equivalent
+SELECT * FROM orders ORDER BY created_at DESC LIMIT 10;
+
+-- Oracle: rows 11–20 (keyset pagination via ROWNUM)
+SELECT * FROM (
+  SELECT t.*, ROWNUM rn FROM (
+    SELECT * FROM orders ORDER BY created_at DESC
+  ) t WHERE ROWNUM <= 20
+) WHERE rn > 10;
+
+-- PostgreSQL equivalent
+SELECT * FROM orders ORDER BY created_at DESC LIMIT 10 OFFSET 10;
+```
+
+## Migration Actions
+
+### 1. Stored Procedures
+
+Replace all `ROWNUM`-based limiting patterns with `LIMIT`/`OFFSET`:
+
+```sql
+-- Oracle
+WHERE ROWNUM = 1
+WHERE ROWNUM <= :n
+
+-- PostgreSQL
+LIMIT 1
+LIMIT :n            -- note: use $n parameter style in PL/pgSQL
+```
+
+For subquery wrapping patterns:
+```sql
+-- Oracle
+SELECT * FROM (SELECT ... ORDER BY col) WHERE ROWNUM <= :n
+
+-- PostgreSQL
+SELECT ... ORDER BY col LIMIT :n
+```
+
+### 2. Application Code (inline SQL strings)
+
+Search for `ROWNUM` in C# string literals, `StringBuilder`, and query-builder methods. Apply the same replacement patterns above.
+
+### 3. Tests
+
+Ensure integration tests validate that result set sizes are correct and that ordering is preserved (i.e., the correct *n* rows are returned, not just any *n* rows).