About Update stats and Indexing Optimize in SQL Server

 Update stats:

Updates query optimization statistics on a table or indexed view. By default, the query optimizer already updates statistics as necessary to improve the query plan; in some cases, you can improve query performance by using UPDATE STATISTICS or the stored procedure sp_updatestats to update statistics more frequently than the default updates.

Updating statistics ensures that queries compile with up-to-date statistics. However, updating statistics causes queries to recompile. We recommend not updating statistics too frequently because there is a performance tradeoff between improving query plans and the time it takes to recompile queries. The specific tradeoffs depend on your application. UPDATE STATISTICS can use tempdb to sort the sample of rows for building statistics.

Indexing Optimize:

SQL Server Indexes are special data structures associated with tables or views that help speed up the query. 

Reorganize or rebuild a fragmented index in SQL Server by using SQL Server Management Studio or Transact-SQL. The SQL Server Database Engine automatically modifies indexes whenever insert, update, or delete operations are made to the underlying data. Over time, these modifications can cause the information in the index to become scattered in the database (fragmented). Fragmentation exists when indexes have pages in which the logical ordering, based on the key value, does not match the physical ordering inside the data file. Heavily fragmented indexes can degrade query performance and cause your application to respond slowly, especially scan operations.

You can remedy index fragmentation by reorganizing or rebuilding an index.