Method addPrewhereAliasActions

src/Interpreters/InterpreterSelectQuery.cpp:2457–2616

}

void InterpreterSelectQuery::addPrewhereAliasActions()
{
    auto & row_level_filter = analysis_result.row_policy_info;
    auto & prewhere_info = analysis_result.prewhere_info;
    auto & columns_to_remove_after_prewhere = analysis_result.columns_to_remove_after_prewhere;

    /// Detect, if ALIAS columns are required for query execution
    auto alias_columns_required = false;
    const ColumnsDescription & storage_columns = metadata_snapshot->getColumns();
    for (const auto & column_name : required_columns)
    {
        auto column_default = storage_columns.getDefault(column_name);
        if (column_default && column_default->kind == ColumnDefaultKind::Alias)
        {
            alias_columns_required = true;
            break;
        }
    }

    /// Set of all (including ALIAS) required columns for PREWHERE
    auto get_prewhere_columns = [&]()
    {
        NameSet columns;

        if (prewhere_info)
        {
            /// Get some columns directly from PREWHERE expression actions
            auto prewhere_required_columns = prewhere_info->prewhere_actions.getRequiredColumns().getNames();
            columns.insert(prewhere_required_columns.begin(), prewhere_required_columns.end());
        }

        if (row_level_filter)
        {
            auto row_level_required_columns = row_level_filter->actions.getRequiredColumns().getNames();
            columns.insert(row_level_required_columns.begin(), row_level_required_columns.end());
        }

        return columns;
    };

    /// There are multiple sources of required columns:
    ///  - raw required columns,
    ///  - columns deduced from ALIAS columns,
    ///  - raw required columns from PREWHERE,
    ///  - columns deduced from ALIAS columns from PREWHERE.
    /// PREWHERE is a special case, since we need to resolve it and pass directly to `IStorage::read()`
    /// before any other executions.
    if (alias_columns_required)
    {
        NameSet required_columns_from_prewhere = get_prewhere_columns();
        NameSet required_aliases_from_prewhere; /// Set of ALIAS required columns for PREWHERE

        /// Expression, that contains all raw required columns
        ASTPtr required_columns_all_expr = make_intrusive<ASTExpressionList>();

        /// Expression, that contains raw required columns for PREWHERE
        ASTPtr required_columns_from_prewhere_expr = make_intrusive<ASTExpressionList>();