Technology to reduce cost of concatenation for hash array

A first table has more than one key column, and a hash array is built for the table, which includes computing hash indices for respective rows of the first table based on more than one key column. Building the hash array also includes writing in the hash array for each row at a respective location in the hash array, wherein each location is indicated by each row's respective hash index computed from the more than one key column. The hash array is probed responsive to the second table. In the building of the hash array for the first table, the writing in the hash array for each row at a respective location in the hash array writes only one of the key columns as a single key field in the hash array for each row.