docs: fix the quickstart flip_activities view

[bosconi]

The quickstart defines a flip as buying an item and reselling it at a higher price "within an 8-day period", but the query never enforced that bound.

datediff(unit, start, end) returns end - start, so datediff('days', w2.bid_time, w1.bid_time) evaluates to purchase_time - sale_time. w1 is the purchase leg and w2 the sale leg, so for any genuine flip this is negative and therefore always < 8.

Two consequences:

• The 8-day window is inert — a resale years after the purchase still qualifies.
• timeframe_days reports a negative number on every genuine flip.

This swaps the arguments so the value is sale - purchase, and adds an explicit ordering predicate, since < 8 alone still admits pairs whose sale precedes the purchase.

Found while investigating memory growth in a demo derived from this quickstart. Worth noting the view is quadratic either way — the fixed window bounds it only once the data spans well beyond 8 days — so this is a correctness fix, not a performance one.

A second commit adds a short note after the index step: the view's output grows with winning_bids, the 8-day bound is what keeps that growth linear rather than quadratic, and an index or materialized view over flip_activities retains that state continuously, so keep the bound or add a temporal filter on winning_bids.

Manual testing

Run in the Materialize Cloud Console SQL shell. Constant queries, no tables involved.

1. Sign convention of datediff:

SELECT
  datediff('days', '2026-01-01'::timestamptz, '2026-01-09'::timestamptz) AS start_then_end,
  datediff('days', '2026-01-09'::timestamptz, '2026-01-01'::timestamptz) AS end_then_start;

start_then_end  end_then_start
8               -8

2. The quickstart's expression applied to a genuine flip (bought Jan 1, sold Jan 5):

WITH flip AS (
  SELECT '2026-01-01 10:00'::timestamptz AS w1_bid_time,   -- purchase leg
         '2026-01-05 10:00'::timestamptz AS w2_bid_time    -- sale leg
)
SELECT
  datediff('days', w2_bid_time, w1_bid_time)      AS timeframe_days_as_written,
  datediff('days', w2_bid_time, w1_bid_time) < 8  AS passes_filter_as_written,
  datediff('days', w1_bid_time, w2_bid_time)      AS timeframe_days_corrected
FROM flip;

timeframe_days_as_written  passes_filter_as_written  timeframe_days_corrected
-4                         true                      4

3. The 8-day bound does nothing (same flip, sold five years later):

SELECT
  datediff('days', '2031-01-01'::timestamptz, '2026-01-01'::timestamptz)      AS timeframe_days_as_written,
  datediff('days', '2031-01-01'::timestamptz, '2026-01-01'::timestamptz) < 8  AS passes_filter_as_written;

timeframe_days_as_written  passes_filter_as_written
-1826                      true

A resale 1,826 days after the purchase passes a filter the prose describes as "within an 8-day period". With the arguments swapped, query 2 reports 4 and query 3 reports 1826, which the < 8 bound then correctly rejects.

🤖 Generated with Claude Code

https://claude.ai/code/session_01W1yEF7rh559n9vxcrSpwWs