⚡ Bolt: [performance improvement] Optimize SQL statement generation in D1ExportContext

.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.
+
+## 2024-05-18 - [Performance: Optimizing SQL string generation]
+**Learning:** Generating SQL statements dynamically in `D1ExportContext` by using intermediate string allocations and vectors (e.g. `vec![]` then `join(", ")`) incurs significant overhead. Direct formatted writes to a pre-allocated `String` using `std::fmt::Write` reduces allocations and drastically improves statement generation performance.
+**Action:** Replace intermediate vectors and string slices with single, pre-allocated strings (`String::with_capacity()`) and `write!()` macros in performance critical SQL statement construction logic.

crates/flow/src/targets/d1.rs

@@ -300,40 +300,52 @@ 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;
+
+        // ⚡ Bolt: Pre-allocate capacity and avoid intermediate vectors
+        let num_keys = key.0.len().min(self.key_fields_schema.len());
+        let num_values = values.fields.len().min(self.value_fields_schema.len());
+        let total_cols = num_keys + num_values;
+
+        let mut params = Vec::with_capacity(total_cols);
+        let mut sql = String::with_capacity(64 + total_cols * 15 + num_values * 30);
+
+        write!(sql, "INSERT INTO {} (", self.table_name).unwrap();
 
-        // Extract key parts - KeyValue is a wrapper around Box<[KeyPart]>
+        let mut first = true;
         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 { sql.push_str(", "); }
+                sql.push_str(&self.key_fields_schema[idx].name);
                 params.push(key_part_to_json(key_part)?);
+                first = false;
             }
         }
 
-        // Add value fields
         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 { sql.push_str(", "); }
+                sql.push_str(&value_field.name);
                 params.push(value_to_json(value)?);
-                update_clauses.push(format!(
-                    "{} = excluded.{}",
-                    value_field.name, value_field.name
-                ));
+                first = false;
             }
         }
 
-        let sql = format!(
-            "INSERT INTO {} ({}) VALUES ({}) ON CONFLICT DO UPDATE SET {}",
-            self.table_name,
-            columns.join(", "),
-            placeholders.join(", "),
-            update_clauses.join(", ")
-        );
+        sql.push_str(") VALUES (");
+        for i in 0..params.len() {
+            if i > 0 { sql.push_str(", "); }
+            sql.push('?');
+        }
+
+        sql.push_str(") ON CONFLICT DO UPDATE SET ");
+        let mut first_update = true;
+        for (idx, _value) in values.fields.iter().enumerate() {
+            if let Some(value_field) = self.value_fields_schema.get(idx) {
+                if !first_update { sql.push_str(", "); }
+                write!(sql, "{0} = excluded.{0}", value_field.name).unwrap();
+                first_update = false;
+            }
+        }
 
         Ok((sql, params))
     }
@@ -342,22 +354,25 @@ 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;
+
+        // ⚡ Bolt: Pre-allocate capacity and avoid intermediate vectors
+        let num_keys = key.0.len().min(self.key_fields_schema.len());
+        let mut params = Vec::with_capacity(num_keys);
 
+        let mut sql = String::with_capacity(32 + num_keys * 20);
+        write!(sql, "DELETE FROM {} WHERE ", self.table_name).unwrap();
+
+        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 { sql.push_str(" AND "); }
+                write!(sql, "{} = ?", self.key_fields_schema[idx].name).unwrap();
                 params.push(key_part_to_json(key_part)?);
+                first = false;
             }
         }
 
-        let sql = format!(
-            "DELETE FROM {} WHERE {}",
-            self.table_name,
-            where_clauses.join(" AND ")
-        );
-
         Ok((sql, params))
     }