HPOS, and reading the migration before enabling it

High-Performance Order Storage moves orders out of the posts table, and the compatibility mode that writes to both is the part to plan around.

the three states:

  legacy         orders in wp_posts. the old world.
  compatibility  both, synchronised. safe, and slower on
                 write. this is where you live during the
                 migration.
  HPOS only      the point of the exercise.

what to check first: every plugin that touches orders,
and every custom query that joins wp_posts on a
shop_order post type. ours had four.

The four queries were the whole cost, and finding them meant grepping for shop_order rather than trusting a compatibility report. Compatibility mode is genuinely safe and genuinely slower, so it is a state to pass through rather than to settle in — the failure mode is a site that stays in it for a year and gets the write cost with none of the read benefit.