Update counter_cache.md

Author: alextanhongpin

counter_cache.md

@@ -174,3 +174,11 @@ alter table ratings add constraint check_rating check(rating between 0 and 5);
 ## Count Estimate
 
 https://wiki.postgresql.org/wiki/Count_estimate
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+counter cache is useful for caching computed single row data.
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+but what if we want to cache bulk data, e,g product ranks, stock quantity or other derived values?
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+we can either create a new table, or use materialized view.
+the latter has an option to just refresh the view, useful if your logic seldom chsnges