update docs

Author: fang-xing-esql

docs/reference/query-languages/esql/esql-multi-index.md

@@ -135,6 +135,54 @@ FROM events_*
 | 2023-10-23T12:27:28.948Z | 172.21.2.113 | 2764889 | Connected to 10.1.0.2 |
 | 2023-10-23T12:15:03.360Z | 172.21.2.162 | 3450233 | Connected to 10.1.0.3 |
 
+When the type of an {{ES|QL}} field is a *union* of `date` and `date_nanos` across different indices, {{ES|QL}} automatically casts all values to the `date_nanos` type during query execution. This implicit casting ensures that all values are handled with nanosecond precision, regardless of their original type. As a result, users can write queries against such fields without needing to perform explicit type conversions, and the query engine will seamlessly align the types for consistent and precise results.
+
+For example, if the `@timestamp` field is mapped as `date` in one index and `date_nanos` in another, {{ES|QL}} will automatically treat all `@timestamp` values as `date_nanos` during query execution. This allows users to write queries that utilize the `@timestamp` field without encountering type mismatch errors, ensuring accurate time-based operations and comparisons across the combined dataset.
+
+**index: events_date**
+
+```
+{
+  "mappings": {
+    "properties": {
+      "@timestamp":     { "type": "date" },
+      "client_ip":      { "type": "ip" },
+      "event_duration": { "type": "long" },
+      "message":        { "type": "keyword" }
+    }
+  }
+}
+```
+
+**index: events_date_nanos**
+
+```
+{
+  "mappings": {
+    "properties": {
+      "@timestamp":     { "type": "date_nanos" },
+      "client_ip":      { "type": "ip" },
+      "event_duration": { "type": "long" },
+      "message":        { "type": "keyword" }
+    }
+  }
+}
+```
+
+```esql
+FROM events_date, events_date_nanos
+| WHERE @timestamp >= "2023-10-23T13:00:00Z"
+| KEEP @timestamp, client_ip, event_duration, message
+| SORT @timestamp DESC
+```
+
+| @timestamp:date | client_ip:ip | event_duration:long | message:keyword |
+| --- | --- | --- | --- |
+| 2023-10-23T13:55:01.543Z | 172.21.3.15 | 1756467 | Connected to 10.1.0.1 |
+| 2023-10-23T13:53:55.832Z | 172.21.3.15 | 5033755 | Connection error |
+| 2023-10-23T13:52:55.015Z | 172.21.3.15 | 8268153 | Connection error |
+| 2023-10-23T13:51:54.732Z | 172.21.3.15 | 725448 | Connection error |
+| 2023-10-23T13:33:34.937Z | 172.21.0.5 | 1232382 | Disconnected |
 
 ## Index metadata [esql-multi-index-index-metadata]