Hash collisions

Dimensions in Adobe Analytics collect string values. Sometimes these strings are hundreds of characters long, while other times they are short. To improve performance, these string values are not used directly in processing. Instead, a hash is computed for each value to make all values a uniform size. All reports run on these hashed values, which drastically increase their performance.

For most fields, the string is first converted to all lowercase. Lowercase conversion reduces the number of unique values. Values are hashed on a monthly basis - the case for a given value uses the first value seen each month. From month to month, there is a small possibility that two unique variable values hash to the same value. This concept is known as a hash collision.

Hash collisions can manifest in reports as follows:

Odds of a hash collision

Adobe Analytics uses 32-bit hashes for most dimensions, which means that there are 2³² possible hash combinations (approximately 4.3 billion). A new hash table for each dimension is created each month. The approximate odds of encountering a hash collision based on the number of unique values are as follows. These odds are based on a single dimension for a single month.

Similar to the birthday paradox, the likelihood of hash collisions drastically increases as the number of unique values increases. At 1 million unique values, it is likely that there are at least 100 hash collisions for that dimension.

Mitigating hash collisions

Most hash collisions happen with two uncommon values, which have no meaningful impact on reports. Even if a hash collides with a common and uncommon value, the result is negligible. However, in rare cases where two popular values experience a hash collision, it is possible to see its effect clearly. Adobe recommends the following to reduce its effect in reports: