⚡ Bolt: [performance improvement] Pre-allocate String and use write! for D1 SQL generation

.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.
+
+## 2026-04-09 - [Performance: Pre-allocate and use write! for dynamic SQL]
+**Learning:** Constructing dynamic SQL queries in hot loops (like D1 target `upsert` and `delete` batching) using `format!` and `Vec::join` creates excessive intermediate string allocations and memory copies.
+**Action:** Use `String::with_capacity` pre-calculated with a conservative estimate, combined with the `write!` macro, to build dynamic SQL statements efficiently without intermediate vectors and heap allocations.

crates/flow/src/targets/d1.rs

@@ -300,40 +300,55 @@ 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());
+        // ⚡ Bolt Optimization: Use String::with_capacity and write! to avoid intermediate Vec<String> and format! allocations
+        let mut sql = String::with_capacity(128 + params.capacity() * 32);
+
+        write!(sql, "INSERT INTO {} (", self.table_name).unwrap();
+        let mut first = true;
 
         // Extract key parts - KeyValue is a wrapper around Box<[KeyPart]>
         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() {
+            sql.push_str(if i > 0 { ", ?" } else { "?" });
+        }
+
+        sql.push_str(") ON CONFLICT DO UPDATE SET ");
+        first = true;
+        for (idx, _value) in values.fields.iter().enumerate() {
+            if let Some(value_field) = self.value_fields_schema.get(idx) {
+                if !first {
+                    sql.push_str(", ");
+                }
+                write!(sql, "{0} = excluded.{0}", value_field.name).unwrap();
+                first = false;
+            }
+        }
 
         Ok((sql, params))
     }
@@ -342,22 +357,24 @@ 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());
+        // ⚡ Bolt Optimization: Use String::with_capacity and write! to avoid intermediate Vec<String> and format! allocations
+        let mut sql = String::with_capacity(32 + self.table_name.len() + params.capacity() * 32);
+        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))
     }