Method allocate

src/QueryPlan/TableScanStep.cpp:1775–1821

}

void TableScanStep::allocate(ContextPtr context)
{
    // init query_info.syntax_analyzer
    if (!query_info.syntax_analyzer_result)
    {
        auto storage_snapshot = storage->getStorageSnapshot(storage->getInMemoryMetadataPtr(), context);
        Block header = storage_snapshot->getSampleBlockForColumns(getRequiredColumns());

        auto tree_rewriter_result
            = std::make_shared<TreeRewriterResult>(header.getNamesAndTypesList(), storage, storage_snapshot);
        tree_rewriter_result->required_source_columns = header.getNamesAndTypesList();
        tree_rewriter_result->analyzed_join = std::make_shared<TableJoin>();
        query_info.syntax_analyzer_result = tree_rewriter_result;
    }

    query_info = fillQueryInfo(context);
    original_table = storage_id.table_name;
    storage_id = storage->prepareTableRead(getRequiredColumns(), query_info, context);
    size_t shards = context->tryGetCurrentWorkerGroup() ? context->getCurrentWorkerGroup()->getShardsInfo().size() : 1;
    if (shards > 1 && !context->getSettingsRef().enable_final_sample)
    {
        ASTSelectQuery * select = query_info.query->as<ASTSelectQuery>();
        if (select && select->sampleSize())
            query_info.query = rewriteSampleForDistributedTable(query_info.query, shards);
    }

    // update query info
    if (query_info.query)
    {
        /// trigger preallocate tables
        // if (!cnch->isOnDemandMode())
        // cnch->read(column_names, query_info, context, processed_stage, max_block_size, 1);
        // cnch->genPlanSegmentQueryAndAllocate(column_names, query_info, context);
        auto db_table = getDatabaseAndTable(query_info.query->as<ASTSelectQuery &>(), 0);
        if (!db_table->table.empty())
        {
            if (db_table->table != storage_id.table_name)
            {
                if (query_info.query)
                {
                    ASTSelectQuery & select_query = query_info.query->as<ASTSelectQuery &>();
                    select_query.replaceDatabaseAndTable(storage_id);
                }
            }
        }
    }
}

bool TableScanStep::hasLimit() const
{