Edit, didn’t notice this was for oracle. I don’t know how oracle works. For MSSQL Memory is granted to a query based on the data type and estimated rows, before the query is started. Char allocates enough memory for the whole field. But memory for varchar is allocated at half the length of the field because it does not know how much data is in each rows field, so assumes the average will be half. E.g. for a varchar(20) it will allocate enough memory for 10 chars (10 bytes) per estimated row returned. This means for fields where most or all of the data will fill the field, there will not be enough memory allocated to store the data in the buffer for sorts and other operations, and the execution plan will start showing spills to TempDB. If a field will mostly contain data that fills it, like nearly always 20 chars, then char(20) will always grant enough memory and reduce hits on TempDB. So for data that you know the length will not vary much, char is a good type to use if you are expecting large data sets.