[release-18.0] 18.0.3 release notes: ExecuteFetchAsDBA breaking change (#15013)

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>
Co-authored-by: Deepthi Sigireddi <deepthi.sigireddi@gmail.com>

Author: shlomi-noach

changelog/18.0/18.0.3/summary.md

@@ -0,0 +1,25 @@
+## Summary
+
+### Table of Contents
+
+- **[Major Changes](#major-changes)**
+  - **[Breaking Changes](#breaking-changes)**
+    - [ExecuteFetchAsDBA rejects multi-statement SQL](#execute-fetch-as-dba-reject-multi)
+
+## <a id="major-changes"/>Major Changes
+
+### <a id="breaking-changes"/>Breaking Changes
+
+#### <a id="execute-fetch-as-dba-reject-multi"/>ExecuteFetchAsDBA rejects multi-statement SQL
+
+`vtctldclient ExecuteFetchAsDBA` (and similarly the `vtctl` and `vtctlclient` commands) now reject multi-statement SQL with error.
+
+For example, `vtctldclient ExecuteFetchAsDBA my-tablet "stop replica; change replication source to auto_position=1; start replica` will return an error, without attempting to execute any of these queries.
+
+Previously, `ExecuteFetchAsDBA` silently accepted multi statement SQL. It would (attempt to) execute all of them, but:
+
+- It would only indicate error for the first statement. Errors on 2nd, 3rd, ... statements were silently ignored.
+- It would not consume the result sets of the 2nd, 3rd, ... statements. It would then return the used connection to the pool in a dirty state. Any further query that happens to take that connection out of the pool could get unexpected results.
+- As another side effect, multi-statement schema changes would cause schema to be reloaded with only the first change, leaving the cached schema inconsistent with the underlying database.
+
+`ExecuteFetchAsDBA` does allow a specific use case of multi-statement SQL, which is where all statements are in the form of `CREATE TABLE` or `CREATE VIEW`. This is to support a common pattern of schema initialization, formalized in `ApplySchema --batch-size` which uses `ExecuteFetchAsDBA` under the hood.

changelog/18.0/README.md

@@ -1,4 +1,6 @@
 ## v18.0
+* **[18.0.3](18.0.3)**
+
 * **[18.0.2](18.0.2)**
 	* [Changelog](18.0.2/changelog.md)
 	* [Release Notes](18.0.2/release_notes.md)