cache_bytes_read and block_byte_read are used to calculate read compression ratios, but they account for different data when a page is read with deltas.

block_byte_read includes the stored bytes of both the base page and its deltas. However, cache_bytes_read includes only the uncompressed bytes of the base page.

This causes the compression calculation to compare base-plus-delta storage bytes against base-only uncompressed bytes.

For example, if a base page is 25 KB stored and 100 KB uncompressed, and its deltas total 5 KB stored and 20 KB uncompressed, the current calculation gives 30 / 100 = 30%. Accounting for both components would give 30 / 120 = 25%.

Update the byte accounting so both statistics cover the corresponding base and delta data, and add regression coverage for reads with deltas.