Merge pull request #4657 from segmentio/lizkane222-patch-6

Add section to Frequently Asked Questions - Profiles Sync

Author: rchinn1

src/unify/profiles-sync/sample-queries.md

@@ -194,3 +194,8 @@ The following edge cases might drive slight (<0.01%) variation:
 - If you rebuild or use non-incremental materialization for `profile_traits`, Profiles Sync will fully calculate traits against a user. As a result, Profiles Sync would ensure that all traits reflect the most recently observed value for fully-merged users.
 
 By contrast, Segment Unify and incrementally-built Profiles Sync materializations won’t combine already-computed traits across two merged profiles at the moment of merge. Instead, one profile’s traits will be chosen across the board.
+
+#### What hash function is used for the external_id_hash field by Profiles Sync?
+
+The `external_id_hash` is a hash of the `external_id_type` and `external_id_value` using SHA-1. This field corresponds to the `primary_key` for the table: `hash (external_id_type and external_id_value)`.
+For example, in BigQuery the logic is: `TO_HEX(SHA1(concat(external_id_type, external_id_value))) as seg_hash`.