internal/postgres: utilize num_imports column in insert_module

- Avoid recomputing number of imports for every query in getUnitWithAllFields.

For golang/go#76284

Change-Id: I7f7d683efb1b2ff49e33681802896a65b1354d25
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/720261
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Ethan Lee <[email protected]>
kokoro-CI: kokoro <[email protected]>

Author: ethanalee-work

internal/postgres/insert_module.go

@@ -409,6 +409,7 @@ func (pdb *DB) insertUnits(ctx context.Context, tx *database.DB,
 			pq.Array(licenseTypes),
 			pq.Array(licensePaths),
 			pdb.bypassLicenseCheck || u.IsRedistributable,
+			len(u.Imports),
 		)
 		if u.Readme != nil {
 			pathToReadme[u.Path] = u.Readme
@@ -481,6 +482,7 @@ func insertUnits(ctx context.Context, db *database.DB, unitValues []any) (pathID
 		"license_types",
 		"license_paths",
 		"redistributable",
+		"num_imports",
 	}
 	uniqueUnitCols := []string{"path_id", "module_id"}
 	returningUnitCols := []string{"id", "path_id"}

internal/postgres/insert_module_test.go

@@ -124,6 +124,7 @@ func checkModule(ctx context.Context, t *testing.T, db *DB, want *internal.Modul
 			}
 		}
 		wantu.Subdirectories = subdirectories
+		wantu.NumImports = len(wantu.Imports)
 		opts := cmp.Options{
 			cmpopts.EquateEmpty(),
 			cmpopts.IgnoreFields(licenses.Metadata{}, "Coverage"),

internal/postgres/unit.go

@@ -474,12 +474,7 @@ func (db *DB) getUnitWithAllFields(ctx context.Context, um *internal.UnitMeta, b
 			r.contents,
 			d.synopsis,
 			d.source,
-			COALESCE((
-				SELECT COUNT(unit_id)
-				FROM imports
-				WHERE unit_id = u.id
-				GROUP BY unit_id
-				), 0) AS num_imports,
+			u.num_imports,
 			COALESCE((
 				SELECT imported_by_count
 				FROM search_documents