Method readIndex

app/src/main/java/androidx/room/TableInfo.java:309–346

@return null if we cannot read the index due to older sqlite implementations.

(SupportSQLiteDatabase database, String name, boolean unique)

     * @return null if we cannot read the index due to older sqlite implementations.
     */
    @Nullable
    private static Index readIndex(SupportSQLiteDatabase database, String name, boolean unique) {
        Cursor cursor = database.query("PRAGMA index_xinfo(`" + name + "`)");
        try {
            final int seqnoColumnIndex = cursor.getColumnIndex("seqno");
            final int cidColumnIndex = cursor.getColumnIndex("cid");
            final int nameColumnIndex = cursor.getColumnIndex("name");
            final int descColumnIndex = cursor.getColumnIndex("desc");
            if (seqnoColumnIndex == -1 || cidColumnIndex == -1
                    || nameColumnIndex == -1 || descColumnIndex == -1) {
                // we cannot read them so better not validate any index.
                return null;
            }
            final TreeMap<Integer, String> columnsMap = new TreeMap<>();
            final TreeMap<Integer, String> ordersMap = new TreeMap<>();

            while (cursor.moveToNext()) {
                int cid = cursor.getInt(cidColumnIndex);
                if (cid < 0) {
                    // Ignore SQLite row ID
                    continue;
                }
                int seq = cursor.getInt(seqnoColumnIndex);
                String columnName = cursor.getString(nameColumnIndex);
                String order = cursor.getInt(descColumnIndex) > 0 ? "DESC" : "ASC";

                columnsMap.put(seq, columnName);
                ordersMap.put(seq, order);
            }
            final List<String> columns = new ArrayList<>(columnsMap.size());
            columns.addAll(columnsMap.values());
            final List<String> orders = new ArrayList<>(ordersMap.size());
            orders.addAll(ordersMap.values());
            return new Index(name, unique, columns, orders);
        } finally {
            cursor.close();
        }
    }

    /**
     * Holds the information about a database column.