[Order 09][P3] Narrow snapshot projection and review snapshot indexes

[AGiorgetti]

Priority order

Order: 09 of 09
Priority: P3 performance

Problem

Snapshot reads use SELECT *, and snapshot indexes should be reviewed after the snapshot-candidate query is fixed.

Evidence:

• src/NEventStore.Persistence.Sql/SqlDialects/CommonSqlStatements.resx:173
• GetSnapshot uses SELECT *.
• Snapshot materialization needs BucketId, StreamId, StreamRevision, and Payload.

Impact

• Current blast radius is low because Snapshots has only four columns.
• Future schema additions could make this hot path read unnecessary data.
• Existing snapshot indexes may not match the corrected query shape from [Order 01][P1] Fix cross-bucket contamination in snapshot candidate query #59.

Mandatory test-first workflow

• Add tests first that verify snapshot materialization still works with explicit projections.
• Use existing snapshot acceptance tests as the behavioral contract.
• Add or document query-plan checks before changing provider indexes.
• Do not change schema/indexes until the expected query behavior is covered.

Proposed action

• Select explicit snapshot columns instead of SELECT *.
• After [Order 01][P1] Fix cross-bucket contamination in snapshot candidate query #59, review whether IX_Snapshots_Stream_Revision should include BucketId first or be replaced by (BucketId, StreamId, StreamRevision) depending on provider plans.

Acceptance criteria

• Snapshot reads return the same objects as before.
• Provider tests pass for the touched SQL dialects.
• Any index change is justified by a query-plan comparison or clear provider-specific reasoning.

Source: docs/Project-Analysis-Issue-Drafts.md.