⚡ Bolt: Optimize dynamic SQL generation in D1 targets

.jules/bolt.md

@@ -2,3 +2,7 @@
 ## 2026-04-08 - [Performance: Defer Allocation during Traversal]
 **Learning:** During DAG traversals, creating owned variants of identifiers (like `file.to_path_buf()`) *before* checking `visited` HashSets results in heap allocations (O(E)) for every edge instead of every visited node (O(V)). By moving the `&PathBuf` allocation strictly *after* all HashSet `contains` checks using the borrowed reference (`&Path`), we drastically reduce memory churn.
 **Action:** Always check `HashSet::contains` with a borrowed reference *before* creating the owned version required by `HashSet::insert`, especially in performance-critical graph traversal paths.
+
+## 2024-05-24 - Dynamic SQL Generation Optimization
+**Learning:** For dynamic SQL generation (e.g., in `crates/flow/src/targets/d1.rs` for Cloudflare D1 targets), using intermediate `Vec` allocations and `format!` in loops is a significant performance bottleneck due to excessive heap allocations and string copies.
+**Action:** Always use `String::with_capacity` and the `write!` macro (via `std::fmt::Write`) to construct queries directly to minimize heap allocations and string copies.

crates/flow/src/targets/d1.rs

@@ -300,40 +300,75 @@ impl D1ExportContext {
         key: &KeyValue,
         values: &FieldValues,
     ) -> Result<(String, Vec<serde_json::Value>), RecocoError> {
-        let mut columns = vec![];
-        let mut placeholders = vec![];
-        let mut params = vec![];
-        let mut update_clauses = vec![];
+        use std::fmt::Write;
+
+        let mut params =
+            Vec::with_capacity(self.key_fields_schema.len() + self.value_fields_schema.len());
+
+        // Pre-calculate string capacity to avoid reallocations
+        // "INSERT INTO " (12) + table_name + " (" (2) + columns + ") VALUES (" (10) + placeholders + ") ON CONFLICT DO UPDATE SET " (28) + update_clauses
+        let estimated_len = 12
+            + self.table_name.len()
+            + 2
+            + (self.key_fields_schema.len() + self.value_fields_schema.len()) * 15
+            + 10
+            + (self.key_fields_schema.len() + self.value_fields_schema.len()) * 3
+            + 28
+            + self.value_fields_schema.len() * 25;
+        let mut sql = String::with_capacity(estimated_len);
 
-        // Extract key parts - KeyValue is a wrapper around Box<[KeyPart]>
-        for (idx, _key_field) in self.key_fields_schema.iter().enumerate() {
+        let _ = write!(sql, "INSERT INTO {} (", self.table_name);
+
+        let mut first = true;
+
+        // Extract key parts
+        for (idx, key_field) in self.key_fields_schema.iter().enumerate() {
             if let Some(key_part) = key.0.get(idx) {
-                columns.push(self.key_fields_schema[idx].name.clone());
-                placeholders.push("?".to_string());
+                if !first {
+                    let _ = write!(sql, ", ");
+                }
+                let _ = write!(sql, "{}", key_field.name);
+                first = false;
                 params.push(key_part_to_json(key_part)?);
             }
         }
 
-        // Add value fields
+        // Add value fields for columns
         for (idx, value) in values.fields.iter().enumerate() {
             if let Some(value_field) = self.value_fields_schema.get(idx) {
-                columns.push(value_field.name.clone());
-                placeholders.push("?".to_string());
+                if !first {
+                    let _ = write!(sql, ", ");
+                }
+                let _ = write!(sql, "{}", value_field.name);
+                first = false;
                 params.push(value_to_json(value)?);
-                update_clauses.push(format!(
-                    "{} = excluded.{}",
-                    value_field.name, value_field.name
-                ));
             }
         }
 
-        let sql = format!(
-            "INSERT INTO {} ({}) VALUES ({}) ON CONFLICT DO UPDATE SET {}",
-            self.table_name,
-            columns.join(", "),
-            placeholders.join(", "),
-            update_clauses.join(", ")
-        );
+        let _ = write!(sql, ") VALUES (");
+
+        // Placeholders
+        for i in 0..params.len() {
+            if i > 0 {
+                let _ = write!(sql, ", ?");
+            } else {
+                let _ = write!(sql, "?");
+            }
+        }
+
+        let _ = write!(sql, ") ON CONFLICT DO UPDATE SET ");
+
+        // Update clauses
+        first = true;
+        for (idx, _value) in values.fields.iter().enumerate() {
+            if let Some(value_field) = self.value_fields_schema.get(idx) {
+                if !first {
+                    let _ = write!(sql, ", ");
+                }
+                let _ = write!(sql, "{name} = excluded.{name}", name = value_field.name);
+                first = false;
+            }
+        }
 
         Ok((sql, params))
     }
@@ -342,22 +377,27 @@ impl D1ExportContext {
         &self,
         key: &KeyValue,
     ) -> Result<(String, Vec<serde_json::Value>), RecocoError> {
-        let mut where_clauses = vec![];
-        let mut params = vec![];
+        use std::fmt::Write;
+
+        let mut params = Vec::with_capacity(self.key_fields_schema.len());
 
-        for (idx, _key_field) in self.key_fields_schema.iter().enumerate() {
+        let estimated_len = 12 + self.table_name.len() + 7 + self.key_fields_schema.len() * 20;
+        let mut sql = String::with_capacity(estimated_len);
+
+        let _ = write!(sql, "DELETE FROM {} WHERE ", self.table_name);
+
+        let mut first = true;
+        for (idx, key_field) in self.key_fields_schema.iter().enumerate() {
             if let Some(key_part) = key.0.get(idx) {
-                where_clauses.push(format!("{} = ?", self.key_fields_schema[idx].name));
+                if !first {
+                    let _ = write!(sql, " AND ");
+                }
+                let _ = write!(sql, "{} = ?", key_field.name);
+                first = false;
                 params.push(key_part_to_json(key_part)?);
             }
         }
 
-        let sql = format!(
-            "DELETE FROM {} WHERE {}",
-            self.table_name,
-            where_clauses.join(" AND ")
-        );
-
         Ok((sql, params))
     }