Stream COPY data directly to temp file to avoid memory buffering

Previously, all COPY data was buffered in memory before writing to
a temp file. For large imports (millions of rows), this could exhaust
memory.

Now we create the temp file upfront and stream data directly to it
as CopyData messages arrive, avoiding memory accumulation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: EDsCODE

server/conn.go

@@ -825,63 +825,59 @@ func (c *clientConn) handleCopyIn(query, upperQuery string) error {
 	c.writer.Flush()
 	log.Printf("[%s] COPY FROM STDIN: sent CopyInResponse, waiting for data...", c.username)
 
-	// Read COPY data from client
-	var allData bytes.Buffer
+	// Create temp file upfront and stream data directly to it (avoids memory buffering)
+	// This approach leverages DuckDB's highly optimized CSV parser which handles
+	// type conversions automatically and can load millions of rows in seconds.
+	tmpFile, err := os.CreateTemp("", "duckgres-copy-*.csv")
+	if err != nil {
+		log.Printf("[%s] COPY FROM STDIN: failed to create temp file: %v", c.username, err)
+		c.sendError("ERROR", "58000", fmt.Sprintf("failed to create temp file: %v", err))
+		c.setTxError()
+		writeReadyForQuery(c.writer, c.txStatus)
+		c.writer.Flush()
+		return nil
+	}
+	tmpPath := tmpFile.Name()
+	defer os.Remove(tmpPath)
+
+	// Stream COPY data directly to temp file (no memory buffering)
 	rowCount := 0
 	copyDataMessages := 0
+	bytesWritten := int64(0)
 	dataReceiveStart := time.Now()
 
 	for {
 		msgType, body, err := readMessage(c.reader)
 		if err != nil {
 			log.Printf("[%s] COPY FROM STDIN: error reading message: %v", c.username, err)
+			tmpFile.Close()
 			return err
 		}
 
 		switch msgType {
 		case msgCopyData:
-			allData.Write(body)
-			copyDataMessages++
-			if copyDataMessages%10000 == 0 {
-				log.Printf("[%s] COPY FROM STDIN: received %d CopyData messages, %d bytes total",
-					c.username, copyDataMessages, allData.Len())
-			}
-
-		case msgCopyDone:
-			dataReceiveElapsed := time.Since(dataReceiveStart)
-			log.Printf("[%s] COPY FROM STDIN: CopyDone received - %d messages, %d bytes in %v",
-				c.username, copyDataMessages, allData.Len(), dataReceiveElapsed)
-
-			// Write data to a temp file and use DuckDB's native COPY FROM for performance.
-			// This approach leverages DuckDB's highly optimized CSV parser which handles
-			// type conversions automatically and can load millions of rows in seconds.
-			// The temp file is created, used, and deleted within this single request.
-			tmpFile, err := os.CreateTemp("", "duckgres-copy-*.csv")
+			n, err := tmpFile.Write(body)
 			if err != nil {
-				log.Printf("[%s] COPY FROM STDIN: failed to create temp file: %v", c.username, err)
-				c.sendError("ERROR", "58000", fmt.Sprintf("failed to create temp file: %v", err))
+				log.Printf("[%s] COPY FROM STDIN: failed to write to temp file: %v", c.username, err)
+				tmpFile.Close()
+				c.sendError("ERROR", "58000", fmt.Sprintf("failed to write to temp file: %v", err))
 				c.setTxError()
 				writeReadyForQuery(c.writer, c.txStatus)
 				c.writer.Flush()
 				return nil
 			}
-			tmpPath := tmpFile.Name()
-			defer os.Remove(tmpPath)
+			bytesWritten += int64(n)
+			copyDataMessages++
+			if copyDataMessages%10000 == 0 {
+				log.Printf("[%s] COPY FROM STDIN: received %d CopyData messages, %d bytes written",
+					c.username, copyDataMessages, bytesWritten)
+			}
 
-			// Write data to temp file
-			writeStart := time.Now()
-			bytesWritten, err := tmpFile.Write(allData.Bytes())
+		case msgCopyDone:
 			tmpFile.Close()
-			if err != nil {
-				log.Printf("[%s] COPY FROM STDIN: failed to write temp file: %v", c.username, err)
-				c.sendError("ERROR", "58000", fmt.Sprintf("failed to write temp file: %v", err))
-				c.setTxError()
-				writeReadyForQuery(c.writer, c.txStatus)
-				c.writer.Flush()
-				return nil
-			}
-			log.Printf("[%s] COPY FROM STDIN: wrote %d bytes to temp file in %v",
-				c.username, bytesWritten, time.Since(writeStart))
+			dataReceiveElapsed := time.Since(dataReceiveStart)
+			log.Printf("[%s] COPY FROM STDIN: CopyDone received - %d messages, %d bytes in %v",
+				c.username, copyDataMessages, bytesWritten, dataReceiveElapsed)
 
 			// Build DuckDB COPY FROM statement
 			// DuckDB syntax: COPY table FROM 'file' (FORMAT CSV, HEADER, NULL 'value', DELIMITER ',')