Cover the eyes-open subquery with an index

The people filter was served from faces_image without touching a row;
reading the eye columns in the same subquery touched every one, and
ALTER TABLE had put those seven floats after the embedding and the crop
blob. One count took 24 seconds on the reference library, thirteen of
them system time. faces_eyes covers the subquery again: five
milliseconds.

core/dr-catalog/src/schema.rs

@@ -15,7 +15,7 @@ use rusqlite::Connection;
 use crate::error::CatalogError;
 
 /// Schema version this build writes and understands.
-pub const SCHEMA_VERSION: i64 = 16;
+pub const SCHEMA_VERSION: i64 = 17;
 
 /// Apply migrations up to [`SCHEMA_VERSION`].
 ///
@@ -160,6 +160,13 @@ pub fn migrate(conn: &Connection) -> Result<i64, CatalogError> {
         tx.commit()?;
     }
 
+    if from < 17 {
+        let tx = conn.unchecked_transaction()?;
+        tx.execute_batch(V17)?;
+        tx.pragma_update(None, "user_version", 17)?;
+        tx.commit()?;
+    }
+
     Ok(from)
 }
 
@@ -744,6 +751,29 @@ CREATE TABLE IF NOT EXISTS xmp_conflicts (
 );
 "#;
 
+const V17: &str = r#"
+-- TRACES: FR-CULL-13 | NFR-P9
+-- The eyes-open filter's index, and a lesson about where a column lands.
+--
+-- The people filter is a correlated EXISTS over `faces` per image, and it
+-- was fast because `faces_image` *covers* it: the subquery never touched a
+-- row. Reading V16's seven eye columns in the same subquery did touch the
+-- row -- and `ALTER TABLE ADD COLUMN` puts a column at the end of the
+-- record, after the 1 KB embedding and the ~5 KB crop, so every check
+-- dragged six kilobytes off disk to reach seven floats. Measured on the
+-- reference library: 24 seconds for one count, thirteen of them system
+-- time. With this index the same count takes five milliseconds, because
+-- the subquery is served from the index again and never reads a row.
+--
+-- The columns are listed in EYE_COLUMNS' order behind `image_id`, which is
+-- the key the subquery searches on. Nothing else changed in V17; a catalog
+-- already at V16 needs only this.
+CREATE INDEX IF NOT EXISTS faces_eyes ON faces(
+    image_id, eye_right, eye_right_px, eye_right_sharp,
+    eye_left, eye_left_px, eye_left_sharp, sunglasses
+);
+"#;
+
 // V16 -- TRACES: FR-CULL-13
 //
 // What each face's eyes are doing: for each eye P(open), the source pixels
@@ -1601,6 +1631,12 @@ mod tests {
         }
         c.pragma_update(None, "user_version", 15).unwrap();
         assert_eq!(migrate(&c).unwrap(), 15);
+        let indexed: bool = c
+            .prepare("SELECT 1 FROM sqlite_master WHERE type = 'index' AND name = 'faces_eyes'")
+            .unwrap()
+            .exists([])
+            .unwrap();
+        assert!(indexed, "V17's covering index is there");
         let v: i64 = c
             .query_row("PRAGMA user_version", [], |r| r.get(0))
             .unwrap();