Alpha numeric selection issue

I have a reporting issue.Report has selection parameter cutomer.

0customer has values numeric and alpha numeric values also(12345590 or RST1200)

When I provide numeric values in the selection parameter 12345590 - 232951090 - Report is executing with in 3 mintues.

When I provide Alpha numeric values in the selection parameter 12345590 - RST1200 .It is taking 3-4 hours to execute the report.

Thanks for your advance help.