keep alias in logical plan

We have been ignoring the alias in the logical plan, but we have to keep
it. Implementing joins in particular is made hard without it, because it
is common that one has the same column name in different tables, just
differentiated by the alias

core/translate/logical.rs

@@ -181,6 +181,7 @@ pub struct Limit {
 #[derive(Debug, Clone, PartialEq)]
 pub struct TableScan {
     pub table_name: String,
+    pub alias: Option<String>,
     pub schema: SchemaRef,
     pub projection: Option<Vec<usize>>, // Column indices to project
 }
@@ -533,7 +534,7 @@ impl<'a> LogicalPlanBuilder<'a> {
     // Build a table reference
     fn build_select_table(&mut self, table: &ast::SelectTable) -> Result<LogicalPlan> {
         match table {
-            ast::SelectTable::Table(name, _alias, _indexed) => {
+            ast::SelectTable::Table(name, alias, _indexed) => {
                 let table_name = Self::name_to_string(&name.name);
                 // Check if it's a CTE reference
                 if let Some(cte_plan) = self.ctes.get(&table_name) {
@@ -545,8 +546,13 @@ impl<'a> LogicalPlanBuilder<'a> {
 
                 // Regular table scan
                 let table_schema = self.get_table_schema(&table_name)?;
+                let table_alias = alias.as_ref().map(|a| match a {
+                    ast::As::As(name) => Self::name_to_string(name),
+                    ast::As::Elided(name) => Self::name_to_string(name),
+                });
                 Ok(LogicalPlan::TableScan(TableScan {
                     table_name,
+                    alias: table_alias,
                     schema: table_schema,
                     projection: None,
                 }))