The deployment list orders `DeployedAt DESC, Id DESC` and joins `Instance` to
make SiteId filterable, but the only DeployedAt index was single-column and
ascending, so the server sorted the filtered set on every page request.
Adds `IX_DeploymentRecords_DeployedAt_Id (DeployedAt DESC, Id DESC)` — the exact
sort key, both columns descending, so a forward scan feeds the paged Skip/Take
directly. Id is in the key as the deterministic tie-break: a bulk site deploy
stamps many rows in the same DeployedAt instant, and without it a row can repeat
or vanish across page boundaries. Three reads share the key
(QueryDeploymentListPageAsync, QueryDeploymentSummariesAsync, and the single-row
GetCurrentDeploymentStatusAsync).
DROPS the old `IX_DeploymentRecords_DeployedAt` rather than keeping both. It is
strictly redundant, not merely overlapping: DeployedAt leads both, so every
predicate it served the composite serves too (direction does not affect
seekability — SQL Server scans either way), and the leaf rows are the same width,
because Id is the clustered PK and was therefore already in the old nonclustered
index's leaf as the row locator. All that changes is Id becoming a declared,
ordered key column. Keeping it would maintain a second index on every deployment
insert and status transition for no read benefit.
Honest caveat: because the clustering key is implicitly ordered within a
nonclustered index, the optimizer may already have been able to serve this
ordering by scanning the old index backward. The explicit composite makes it
deterministic and self-documenting rather than dependent on that inference, and
costs nothing to declare.
Migration 20260815165452_AddDeploymentRecordListSortIndex (scaffolded with a real
build, non-empty, snapshot updated) plus the hand-written production script under
docs/plans/sql/ following the AddAuditLogAndNotificationCoveringIndexes
precedent. SystemArtifactDeploymentRecords has its own separate DeployedAt index
and is deliberately untouched.
Joseph Doherty
be29bf471e