mirror of https://github.com/sysown/proxysql
Rene Cannao
2 months ago
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# ProxySQL Query Rules: Capture Groups and Backreferences
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## Introduction
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ProxySQL's query rules engine supports regular expression capture groups and backreferences, allowing sophisticated query rewriting. This document explains how to use these features to transform SQL queries dynamically.
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## Core Concepts
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### Table Columns for Pattern Matching
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| Column | Purpose | Example |
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|--------|---------|---------|
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| `digest` | Hash of the normalized query pattern | `0x1d2cc217c860282` |
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| `match_digest` | Normalized query pattern with placeholders | `SELECT * FROM users WHERE id = ?` |
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| `match_pattern` | Raw query pattern with regex groups | `SELECT (.*) FROM users WHERE id = (\d+)` |
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| `replace_pattern` | Replacement pattern with backreferences | `SELECT \1 FROM customers WHERE user_id = \2` |
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| `re_modifiers` | Regex modifiers | `'CASELESS'` or `'CASELESS,GLOBAL'` |
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### Regex Engine Support
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ProxySQL supports two regex engines (configurable via `mysql-query_processor_regex`):
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- **PCRE** (default, `query_processor_regex=1`): Full regex support including capture groups
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- **RE2** (`query_processor_regex=2`): Google's RE2 library, supports capture groups in replacement patterns
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Both engines support backreferences (`\1`, `\2`, etc.) in `replace_pattern`.
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## Basic Syntax
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### Capture Groups in match_pattern
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Use parentheses `()` to define capture groups:
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```sql
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-- Two capture groups: column list and WHERE clause
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INSERT INTO mysql_query_rules (
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match_pattern, replace_pattern, apply
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) VALUES (
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'SELECT (.*) FROM users WHERE (.*)',
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'SELECT \1 FROM customers WHERE \2',
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1
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);
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```
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### Backreferences in replace_pattern
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Reference captured groups with `\1`, `\2`, etc.:
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```sql
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-- \1 = column list, \2 = WHERE conditions
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'\1 FROM modified_table WHERE \2'
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```
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**Important**: Use single backslash (`\1`), not double (`\\1`), in the SQL INSERT statement.
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## Practical Examples
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### Example 1: Changing Table Names While Preserving Query Structure
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**Goal**: Rewrite queries from `old_table` to `new_table` while keeping all other parts unchanged.
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```sql
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INSERT INTO mysql_query_rules (
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rule_id, active, match_pattern, replace_pattern, re_modifiers, apply
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) VALUES (
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1, 1,
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'(SELECT .* FROM )old_table( WHERE .*)',
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'\1new_table\2',
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'CASELESS',
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1
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);
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```
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**Matches**: `SELECT id, name FROM old_table WHERE status = 'active'`
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**Becomes**: `SELECT id, name FROM new_table WHERE status = 'active'`
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### Example 2: Adding Hints to Specific Queries
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**Goal**: Add `FORCE INDEX (primary)` to SELECT queries on `orders` table with specific conditions.
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```sql
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INSERT INTO mysql_query_rules (
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rule_id, active, digest, match_pattern, replace_pattern, apply
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) VALUES (
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2, 1, '0x1234567890abcdef',
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'(SELECT .* FROM orders)( WHERE customer_id = \d+.*)',
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'\1 FORCE INDEX (primary)\2',
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1
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);
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```
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**Matches**: `SELECT * FROM orders WHERE customer_id = 100 AND date > '2024-01-01'`
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**Becomes**: `SELECT * FROM orders FORCE INDEX (primary) WHERE customer_id = 100 AND date > '2024-01-01'`
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### Example 3: Column Renaming in SELECT Queries
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**Goal**: Rename column `legacy_id` to `new_id` in all SELECT queries.
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```sql
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INSERT INTO mysql_query_rules (
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rule_id, active, match_pattern, replace_pattern, re_modifiers, apply
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) VALUES (
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3, 1,
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'(SELECT.*?)legacy_id(.*FROM.*)',
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'\1new_id\2',
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'CASELESS',
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1
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);
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```
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**Matches**: `SELECT legacy_id, name FROM products WHERE category = 'electronics'`
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**Becomes**: `SELECT new_id, name FROM products WHERE category = 'electronics'`
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### Example 4: Conditional Rewriting Based on Values
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**Goal**: Add `USE INDEX` hint only for queries with `status = 'pending'`.
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```sql
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INSERT INTO mysql_query_rules (
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rule_id, active, match_pattern, replace_pattern, re_modifiers, apply
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) VALUES (
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4, 1,
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'(SELECT .* FROM tasks)( WHERE.*status\s*=\s*''pending''.*)',
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'\1 USE INDEX (idx_status)\2',
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'CASELESS',
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1
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);
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```
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**Matches**: `SELECT * FROM tasks WHERE status = 'pending' AND due_date < NOW()`
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**Becomes**: `SELECT * FROM tasks USE INDEX (idx_status) WHERE status = 'pending' AND due_date < NOW()`
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### Example 5: Complex Multi-Group Rewriting
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**Goal**: Reorder WHERE clause conditions and add optimizer hint.
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```sql
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INSERT INTO mysql_query_rules (
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rule_id, active, match_pattern, replace_pattern, re_modifiers, apply
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) VALUES (
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5, 1,
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'SELECT (.*) FROM (\w+) WHERE (column1 = .*) AND (column2 = .*)',
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'SELECT /*+ MAX_EXECUTION_TIME(1000) */ \1 FROM \2 WHERE \4 AND \3',
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'CASELESS',
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1
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);
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```
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**Matches**: `SELECT id, name FROM accounts WHERE column1 = 'value1' AND column2 = 'value2'`
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**Becomes**: `SELECT /*+ MAX_EXECUTION_TIME(1000) */ id, name FROM accounts WHERE column2 = 'value2' AND column1 = 'value1'`
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## Advanced Techniques
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### Combining digest and match_pattern
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For precise targeting, combine `digest` (hash of normalized query) with `match_pattern` (specific values):
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```sql
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INSERT INTO mysql_query_rules (
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rule_id, active, digest, match_pattern, replace_pattern, apply
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) VALUES (
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6, 1, '0xa1b2c3d4e5f67890',
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'(SELECT .* FROM users)( WHERE id = 12345.*)',
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'\1 FORCE INDEX (primary)\2',
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1
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);
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```
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### Using re_modifiers
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- `CASELESS`: Case-insensitive matching
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- `GLOBAL`: Replace all occurrences (not just first)
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```sql
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INSERT INTO mysql_query_rules (
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rule_id, active, match_pattern, re_modifiers, replace_pattern, apply
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) VALUES (
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7, 1,
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'(SELECT)(.*)(FROM)(.*)',
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'CASELESS,GLOBAL',
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'\1 SQL_NO_CACHE \2\3\4',
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1
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);
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```
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### Rule Chaining with flagIN/flagOUT
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For complex transformations, chain rules using flags:
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```sql
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-- Rule 1: Match pattern and set flagOUT
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INSERT INTO mysql_query_rules (
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rule_id, active, match_pattern, flagOUT, apply
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) VALUES (
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8, 1, 'SELECT .* FROM sensitive_table', 100, 0
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);
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-- Rule 2: Apply transformation only when flagIN matches
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INSERT INTO mysql_query_rules (
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rule_id, active, flagIN, match_pattern, replace_pattern, apply
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) VALUES (
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9, 1, 100,
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'(SELECT .* FROM )sensitive_table( WHERE .*)',
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'\1audited_sensitive_table\2',
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1
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);
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```
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## Testing and Validation
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### 1. Verify Rule Matching
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```sql
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-- Check stats for specific rule
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SELECT * FROM stats_mysql_query_rules WHERE rule_id = 1;
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-- Test pattern matching
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SELECT * FROM mysql_query_rules
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WHERE match_pattern = '(SELECT .* FROM )old_table( WHERE .*)';
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```
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### 2. Monitor Rule Performance
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```sql
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-- View hits and performance
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SELECT rule_id, hits, mysql_query_rules.match_pattern,
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sum_time, min_time, max_time
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FROM stats_mysql_query_rules
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JOIN mysql_query_rules USING (rule_id)
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ORDER BY hits DESC;
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```
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## Common Pitfalls and Solutions
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### Problem 1: Groups Not Capturing Entire Needed Text
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**Symptom**: Replacement loses part of the original query.
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**Solution**: Expand capture groups to include more context:
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```sql
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-- Before (loses WHERE clause):
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'(SELECT .* FROM table)( WHERE)'
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-- After (captures entire WHERE clause):
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'(SELECT .* FROM table)( WHERE.*)'
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```
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### Problem 2: Backreferences Not Working
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**Symptom**: `\1` appears literally in output instead of replaced text.
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**Solution**: Ensure:
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1. Parentheses in `match_pattern` define capture groups
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2. `replace_pattern` uses `\1`, not `\\1` or `$1`
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3. Rule is active (`active = 1`)
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### Problem 3: Overly Broad Matching
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**Symptom**: Rule applies to unintended queries.
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**Solution**: Add more specific constraints:
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- Use `digest` column to restrict to specific query patterns
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- Add `username` or `schemaname` restrictions
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- Make `match_pattern` more specific
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```sql
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INSERT INTO mysql_query_rules (
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active, username, digest, match_pattern, replace_pattern, apply
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) VALUES (
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1, 'app_user', '0x1234567890abcdef',
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'(SELECT .* FROM orders)( WHERE .*)',
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'\1 FORCE INDEX (primary)\2',
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1
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);
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```
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## Best Practices
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1. **Test Incrementally**: Start with simple patterns, then add complexity.
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2. **Use digest for Precision**: Combine `digest` with `match_pattern` for exact targeting.
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3. **Case-Insensitive by Default**: Use `re_modifiers = 'CASELESS'` unless case sensitivity is required.
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4. **Monitor Performance**: Regularly check `stats_mysql_query_rules` for rule hits and timing.
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5. **Document Rules**: Add comments to rules explaining their purpose:
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```sql
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INSERT INTO mysql_query_rules (
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rule_id, active, match_pattern, replace_pattern, apply, comment
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) VALUES (
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10, 1,
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'(SELECT .* FROM )products( WHERE .*)',
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'\1products_v2\2',
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1,
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'Rewrite: Route queries from products to products_v2 table'
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);
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```
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6. **Version Control**: Keep query rule definitions in version-controlled SQL files.
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## Conclusion
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ProxySQL's capture group and backreference capabilities provide powerful query rewriting options. By understanding how to properly structure `match_pattern` with parentheses and reference captured groups with `\1`, `\2` in `replace_pattern`, you can implement sophisticated query transformations while maintaining query correctness.
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Always test rules in a staging environment before deploying to production, and monitor their impact on query performance and correctness.
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