improve efficiency of dim_ga4__sessions_daily

[dgitis]

Description & motivation

Replaced FIRST_VALUE() OVER (window) + DISTINCT with qualify row_number() = 1.

This matches how dim_ga4__sessions works and is more efficient. The previous approach was causing memory issues with one client while this approach works for them.

Why the old approach was expensive:

• The window function computed the "first value" for every row in the partition, attaching a copy of the result to each row
• All those rows (with duplicated first-values) were materialized in memory as an intermediate result
• Then DISTINCT made a second pass to collapse them back down

So for a session with 50 events, BigQuery materialized 50 identical rows just to throw away 49.

The qualify approach: BigQuery sorts each partition, assigns row numbers, and filters to keep only row 1 — no intermediate blowup, no second dedup pass.

Checklist

• [ y ] I have verified that these changes work locally
• [ n/a ] I have updated the README.md (if applicable)
• [ n/a ] I have added tests & descriptions to my models (and macros if applicable)
• [ y ] I have run dbt test and python -m pytest . to validate existing tests

[michaelf108]

Hi, I noticed the same window + DISTINCT pattern is in stg_ga4__client_key_first_last_events too, just doubled up: it uses FIRST_VALUE and LAST_VALUE for the first and last event, then DISTINCT, then joins back to stg_ga4__events twice to pull the geo/device/source columns onto each side.

I've made a version that moves it over to the same qualify row_number() = 1 pattern you used in #392. Two CTEs (first event, last event) that keep the whole row, joined on client_key, so both self-joins and the DISTINCT drop out, same memory optimisation you described there. Same columns, grain, and one row per client_key, and it passes the existing .py test in the repo.

Happy to open a PR if that's useful. If not, no problem, happy to wait until #392 lands and align with that!

[mtcarlone]

not sure if this a real PR, but lgtm

[mtcarlone]

If this is live, then let's remove the draft status and I can release this on Sept. 8