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76 lines
2.1 KiB
Markdown
76 lines
2.1 KiB
Markdown
# Oracle to PostgreSQL: ROWNUM Pagination vs LIMIT/OFFSET
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## Problem
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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.
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## Behavior Comparison
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**Oracle:**
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- `ROWNUM` is assigned before `ORDER BY` — filtering with `ROWNUM` on an unordered result set is non-deterministic
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- Common pattern to get "top N ordered rows" requires a subquery:
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```sql
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SELECT * FROM (SELECT * FROM t ORDER BY col) WHERE ROWNUM <= 10
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```
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- `ROWNUM BETWEEN n AND m` requires a double-wrapped subquery
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**PostgreSQL:**
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- `LIMIT n` restricts result rows after `ORDER BY` is applied — straightforward and deterministic
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- `OFFSET n` skips rows; combine with `LIMIT` for pagination
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- No `ROWNUM` pseudo-column exists
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## Code Example
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```sql
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-- Oracle: top 10 rows by date
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SELECT * FROM (
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SELECT * FROM orders ORDER BY created_at DESC
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) WHERE ROWNUM <= 10;
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-- PostgreSQL equivalent
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SELECT * FROM orders ORDER BY created_at DESC LIMIT 10;
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-- Oracle: rows 11–20 (keyset pagination via ROWNUM)
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SELECT * FROM (
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SELECT t.*, ROWNUM rn FROM (
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SELECT * FROM orders ORDER BY created_at DESC
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) t WHERE ROWNUM <= 20
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) WHERE rn > 10;
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-- PostgreSQL equivalent
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SELECT * FROM orders ORDER BY created_at DESC LIMIT 10 OFFSET 10;
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```
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## Migration Actions
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### 1. Stored Procedures
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Replace all `ROWNUM`-based limiting patterns with `LIMIT`/`OFFSET`:
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```sql
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-- Oracle
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WHERE ROWNUM = 1
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WHERE ROWNUM <= :n
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-- PostgreSQL
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LIMIT 1
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LIMIT :n -- note: use $n parameter style in PL/pgSQL
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```
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For subquery wrapping patterns:
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```sql
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-- Oracle
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SELECT * FROM (SELECT ... ORDER BY col) WHERE ROWNUM <= :n
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-- PostgreSQL
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SELECT ... ORDER BY col LIMIT :n
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```
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### 2. Application Code (inline SQL strings)
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Search for `ROWNUM` in C# string literals, `StringBuilder`, and query-builder methods. Apply the same replacement patterns above.
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### 3. Tests
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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).
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