Only load the column names for queries executed by Connection::load()

We don't need to have column names for statements where we will only
receive the number of affected rows. This should hopefully remove the
performance impact of the previous fix for insert statements

Author: weiznich

diesel/src/sqlite/connection/mod.rs

@@ -83,7 +83,7 @@ impl Connection for SqliteConnection {
         Self::Backend: QueryMetadata<T::SqlType>,
     {
         let mut statement = self.prepare_query(&source.as_query())?;
-        let statement_use = StatementUse::new(&mut statement);
+        let statement_use = StatementUse::new(&mut statement, true);
         let iter = StatementIterator::new(statement_use);
         iter.collect()
     }
@@ -94,7 +94,7 @@ impl Connection for SqliteConnection {
         T: QueryFragment<Self::Backend> + QueryId,
     {
         let mut statement = self.prepare_query(source)?;
-        let mut statement_use = StatementUse::new(&mut statement);
+        let mut statement_use = StatementUse::new(&mut statement, false);
         statement_use.run()?;
         Ok(self.raw_connection.rows_affected_by_last_query())
     }

diesel/src/sqlite/connection/stmt.rs

@@ -117,15 +117,20 @@ impl Drop for Statement {
 pub struct StatementUse<'a: 'b, 'b> {
     statement: &'a mut Statement,
     column_names: Vec<&'b str>,
+    should_init_column_names: bool,
 }
 
 impl<'a, 'b> StatementUse<'a, 'b> {
-    pub(in crate::sqlite::connection) fn new(statement: &'a mut Statement) -> Self {
+    pub(in crate::sqlite::connection) fn new(
+        statement: &'a mut Statement,
+        should_init_column_names: bool,
+    ) -> Self {
         StatementUse {
             statement,
             // Init with empty vector because column names
             // can change till the first call to `step()`
             column_names: Vec::new(),
+            should_init_column_names,
         }
     }
 
@@ -146,10 +151,11 @@ impl<'a, 'b> StatementUse<'a, 'b> {
                 _ => Err(last_error(self.statement.raw_connection())),
             }
         }?;
-        if self.column_names.is_empty() {
+        if self.should_init_column_names {
             self.column_names = (0..self.column_count())
                 .map(|idx| unsafe { self.column_name(idx) })
                 .collect();
+            self.should_init_column_names = false;
         }
         Ok(res.map(move |()| SqliteRow::new(self)))
     }