Calculating storage requirements for in-memory tables
The general formula for the space required for an in-memory table is:
table_size + sum_of(index_sizes)
table_size = 1.3 x rows x (sum_of(col_sizes) + (3 x word_size) + (2 * num_cols) + 2)
where:
rows is the number of rows
word_size is the machine word size (for example, 4 bytes for 32-bit OS and 8 bytes for 64-bit OS)
num_cols is the number of columns
sum_of(col_sizes) is the sum of the sizes of the columns
For each in-memory index, the index size is:
1.3 x rows x ((dist_factor x sum_of(col_sizes + 1)) + (8 x word_size) + 4)
where dist_factor is a value between 1.0 and 2.0 that depends upon the distribution of the key values. If key values are highly dissimilar, use a value closer to 2.0. If key values are highly similar, use a value closer to 1.0.