delta datagram size estimation

Author: jerrinot

TODO.md

@@ -0,0 +1,32 @@
+# TODO
+
+## QWP UDP Sender
+
+### Documented limitation: mixing `atNow()` with `atMicros()` / `atNanos()`
+
+Current behavior in `QwpUdpSender` is to reject this pattern once committed rows already exist for the table:
+
+1. Write row(s) with `atNow()` (server-assigned designated timestamp).
+2. Start a later row and finish it with `atMicros()` or `atNanos()`.
+
+The sender throws:
+
+- `schema change in middle of row is not supported`
+
+Why this happens:
+
+- `atNow()` does not write the designated timestamp column.
+- `atMicros()` / `atNanos()` writes designated timestamp into the empty-name column (`""`).
+- With committed rows already present, introducing this column is treated as schema evolution.
+- The UDP incremental-estimate policy forbids schema changes in the middle of an in-progress row.
+
+Current workaround:
+
+- Use one designated timestamp strategy consistently per table stream:
+  - always `atNow()`, or
+  - always `atMicros()` / `atNanos()`.
+
+Future fix options:
+
+- Add explicit support for switching designated timestamp strategy mid-stream by pre-materializing designated timestamp schema state, or
+- Harmonize designated timestamp handling so `atNow()` and `atMicros()` / `atNanos()` do not diverge schema shape.

core/src/main/java/io/questdb/client/cutlass/qwp/client/NativeBufferWriter.java

@@ -81,6 +81,17 @@ public static int utf8Length(String s) {
         return len;
     }
 
+    /**
+     * Returns the number of bytes required to encode {@code value} as an
+     * unsigned LEB128 varint.
+     */
+    public static int varintSize(long value) {
+        if (value == 0) {
+            return 1;
+        }
+        return (64 - Long.numberOfLeadingZeros(value) + 6) / 7;
+    }
+
     @Override
     public void close() {
         if (bufferPtr != 0) {