Optimizing Query Text Agent Skill

Optimize Query from SQL Text

OUTPUT FORMAT

Return ONLY the optimized SQL query. No markdown formatting, no explanations, no bullet points - just pure SQL that can be executed directly in Snowflake.

CRITICAL: Semantic Preservation Rules

The optimized query MUST return IDENTICAL results to the original.

Before returning ANY optimization, verify:

• Same columns: Exact same columns in exact same order with exact same aliases
• Same rows: Filter conditions must be semantically equivalent
• Same ordering: Preserve ORDER BY exactly as written
• Same limits: If original has LIMIT N, keep LIMIT N. If no LIMIT, do NOT add one.

If you cannot guarantee identical results, return the original query unchanged.

Pattern 1: Function on Filter Column

Problem: Functions on columns in WHERE clause prevent partition pruning and index usage.

CAN Fix

| Original | Optimized | Why Safe |

|----------|-----------|----------|

| WHERE DATE(ts) = '2024-01-01' | WHERE ts >= '2024-01-01' AND ts < '2024-01-02' | Equivalent range |

| WHERE YEAR(dt) = 2024 | WHERE dt >= '2024-01-01' AND dt < '2025-01-01' | Equivalent range |

| WHERE MONTH(dt) = 3 AND YEAR(dt) = 2024 | WHERE dt >= '2024-03-01' AND dt < '2024-04-01' | Equivalent range |

| WHERE DATE(ts) >= '2024-01-01' AND DATE(ts) < '2024-02-01' | WHERE ts >= '2024-01-01' AND ts < '2024-02-01' | Same boundaries |

| WHERE YEAR(dt) BETWEEN 1995 AND 1996 | WHERE dt >= '1995-01-01' AND dt < '1997-01-01' | Equivalent range |

CANNOT Fix

| Pattern | Why Not |

|---------|---------|

| WHERE YEAR(dt) IN (SELECT year FROM ...) | Dynamic values, cannot precompute range |

| WHERE DATE(ts) = DATE(other_col) | Comparing two columns, both need function |

| WHERE EXTRACT(DOW FROM dt) = 1 | Day-of-week has no contiguous range |

| WHERE DATE_TRUNC('month', dt) = '2024-01-01' in GROUP BY | Needed for grouping logic |

| SELECT YEAR(dt) AS yr ... GROUP BY YEAR(dt) | Function in SELECT/GROUP BY is fine, only filter matters |

Pattern 2: Function on JOIN Column

Problem: Functions on JOIN columns prevent hash joins, forcing slower nested loop joins.

CAN Fix

| Original | Optimized | Why Safe |

|----------|-----------|----------|

| ON CAST(a.id AS VARCHAR) = CAST(b.id AS VARCHAR) | ON a.id = b.id | If both are same type (e.g., INTEGER) |

| ON UPPER(a.code) = UPPER(b.code) | ON a.code = b.code | If data is already consistently cased |

| ON TRIM(a.name) = TRIM(b.name) | ON a.name = b.name | If data has no leading/trailing spaces |

CANNOT Fix

| Pattern | Why Not |

|---------|---------|

| ON CAST(a.id AS VARCHAR) = b.string_id | Types genuinely differ, CAST required |

| ON DATE(a.timestamp) = b.date_col | Different granularity, DATE() required |

| ON UPPER(a.code) = b.code | If b.code might have different case |

| ON a.id = b.id + 1 | Arithmetic transformation, cannot remove |

Pattern 3: NOT IN Subquery

Problem: NOT IN has poor performance and unexpected NULL behavior.

CAN Fix

| Original | Optimized | Why Safe |

|----------|-----------|----------|

| WHERE id NOT IN (SELECT id FROM t WHERE ...) | WHERE NOT EXISTS (SELECT 1 FROM t WHERE t.id = main.id AND ...) | Equivalent when subquery column is NOT NULL |

| WHERE id NOT IN (SELECT id FROM t) where id has NOT NULL constraint | WHERE NOT EXISTS (SELECT 1 FROM t WHERE t.id = main.id) | NOT NULL guarantees equivalence |

CANNOT Fix

| Pattern | Why Not |

|---------|---------|

| WHERE id NOT IN (SELECT nullable_col FROM t) | If subquery returns NULL, NOT IN returns no rows; NOT EXISTS doesn't |

| WHERE (a, b) NOT IN (SELECT x, y FROM t) | Multi-column NOT IN has complex NULL semantics |

Key Rule: Only convert NOT IN to NOT EXISTS if you can verify the subquery column cannot be NULL.

Pattern 4: Repeated Subquery

Problem: Same subquery executed multiple times causes redundant scans.

CAN Fix

| Original | Optimized |

|----------|-----------|

| Subquery appears 2+ times identically | Extract to CTE, reference CTE multiple times |

| Same aggregation used in multiple places | Compute once in CTE |

CANNOT Fix

| Pattern | Why Not |

|---------|---------|

| Correlated subquery (references outer table) | Each execution is different, cannot cache |

| Subqueries with different filters | Not actually the same subquery |

| Subquery in SELECT that depends on current row | Correlation prevents extraction |

Pattern 5: Implicit Comma Joins

Problem: Comma-separated tables in FROM clause are harder to read and optimize.

CAN Fix - Always

Convert FROM a, b, c WHERE a.id = b.id AND b.id = c.id to explicit JOIN syntax.

This is always safe - just restructuring, no semantic change.

UNSAFE Optimizations (NEVER apply)

• UNION to UNION ALL: UNION deduplicates rows, UNION ALL does not - different results
• Changing window functions: Do not modify SUM(SUM(x)) OVER(...) or similar nested aggregates
• Adding redundant filters: Do not add filters in JOIN ON if same filter exists in WHERE
• Changing column names: Copy column names EXACTLY from original - do not "simplify" or rename
• Changing column aliases: Keep all aliases exactly as original
• Adding early filtering in JOINs: If a filter is in WHERE, do not duplicate it in JOIN ON clause

Principles

• Minimal changes: Make the fewest changes necessary. Simpler optimizations are more reliable.
• Preserve structure: Keep subqueries, CTEs, and overall query structure unless there's a clear benefit.
• When in doubt, don't: If unsure whether a change preserves semantics, skip it.
• Copy exactly: Column names, table aliases, and expressions should be copied character-for-character.

Priority Order

• Date/time functions on filter columns - Highest impact
• Implicit joins to explicit JOIN - Always safe, improves readability
• NOT IN to NOT EXISTS - Only if NULL-safe

Requirements

• Results must be identical: Same rows, same columns, same order
• Valid Snowflake SQL: Output must execute without errors in Snowflake