topology: De-duplicate query_tables_schema calls

This function performs a request that fetches columns for all tables
and views. It is potentially the most performance-impactful part of the
schema fetching process, and it was unnecessarily called twice:
in query_tables and in query_views.

It can be easily prevented by calling the function earlier and passing
the result to query_tables and query_views.

Author: Lorak-mmk

scylla/src/cluster/metadata.rs

@@ -1012,9 +1012,17 @@ async fn query_keyspaces(
 
     let (mut all_tables, mut all_views, mut all_user_defined_types) = if fetch_schema {
         let udts = query_user_defined_types(conn, keyspaces_to_fetch).await?;
+        let mut tables_schema = query_tables_schema(conn, keyspaces_to_fetch, &udts).await?;
         (
-            query_tables(conn, keyspaces_to_fetch, &udts).await?,
-            query_views(conn, keyspaces_to_fetch, &udts).await?,
+            // We pass the mutable reference to the same map to the both functions.
+            // First function fetches `system_schema.tables`, and removes found
+            // table from `tables_schema`.
+            // Second does the same for `system_schema.views`.
+            // The assumption here is that no keys (table names) can appear in both
+            // of those schema table.
+            // As far as we know this assumption is true for Scylla and Cassandra.
+            query_tables(conn, keyspaces_to_fetch, &mut tables_schema).await?,
+            query_views(conn, keyspaces_to_fetch, &mut tables_schema).await?,
             udts,
         )
     } else {
@@ -1411,7 +1419,7 @@ mod toposort_tests {
 async fn query_tables(
     conn: &Arc<Connection>,
     keyspaces_to_fetch: &[String],
-    udts: &HashMap<String, HashMap<String, Arc<UserDefinedType>>>,
+    tables: &mut HashMap<(String, String), Table>,
 ) -> Result<HashMap<String, HashMap<String, Table>>, QueryError> {
     let rows = query_filter_keyspace_name::<(String, String)>(
         conn,
@@ -1420,7 +1428,6 @@ async fn query_tables(
         |err| MetadataError::Tables(TablesMetadataError::SchemaTablesInvalidColumnType(err)),
     );
     let mut result = HashMap::new();
-    let mut tables = query_tables_schema(conn, keyspaces_to_fetch, udts).await?;
 
     rows.map(|row_result| {
         let keyspace_and_table_name = row_result?;
@@ -1448,7 +1455,7 @@ async fn query_tables(
 async fn query_views(
     conn: &Arc<Connection>,
     keyspaces_to_fetch: &[String],
-    udts: &HashMap<String, HashMap<String, Arc<UserDefinedType>>>,
+    tables: &mut HashMap<(String, String), Table>,
 ) -> Result<HashMap<String, HashMap<String, MaterializedView>>, QueryError> {
     let rows = query_filter_keyspace_name::<(String, String, String)>(
         conn,
@@ -1458,7 +1465,6 @@ async fn query_views(
     );
 
     let mut result = HashMap::new();
-    let mut tables = query_tables_schema(conn, keyspaces_to_fetch, udts).await?;
 
     rows.map(|row_result| {
         let (keyspace_name, view_name, base_table_name) = row_result?;