I have a table with a multicolumn index, and I have doubts about the proper sorting of the indexes to get the maximum performance on the queries.
PostgreSQL 8.4, table with about one million rows
Values in column c1 can have about 100 different values. We can assume the values are evenly distributed, so we have about 10000 rows for every possible value.
Column c2 can have 1000 different values. We have 1000 rows for every possible value.
When searching data, the condition always includes values for these two columns, so the table has a multicolumn index combining c1 and c2. I have read about the importance of properly ordering the columns in a multicolumn index if you have queries using just one column for filtering. This is not the case in our scenario.
My question is this one:
Given the fact that one of the filters selects a much smaller set of data, could I improve performance if the first index is the most selective one (the one which allows a smaller set)? I had never considered this question until I saw the graphics from the referenced article:
Image taken from the referenced article about multicolumn indexes.
The queries use values from the two columns for filtering. I have no queries using just one column for filtering. All of them are:
WHERE c1=@ParameterA AND c2=@ParameterB. There are also conditions like this:
WHERE c1 = "abc" AND c2 LIKE "ab%"