Large Datatype length performance impact in Oracle?

Question

I am adding a column with datatype varchar2(1000), This column will be used to store a large set of message(approximately (600 characters).Does it effect the performance of query for having large datatype length, if so how? I will be having a query selecting that column occasionally. Does a table consume extra memory here even if the value in that field in some places 100 characters?

Answer 1

Does it affect performance? It depends.

If "adding a column" implies that you have an existing table with existing data that you're adding a new column to, are you going to populate the new column for old data? If so, depending on your PCTFREE settings and the existing size of the rows, increasing the size of every row by an average of 600 bytes could well lead to row migration which could potentially increase the amount of I/O that queries need to perform to fetch a row. You may want to create a new table with the new column and move the old data to the new table while simultaneously populating the new column if this is a concern.

If you have queries that involve full table scans on the table, anything that you do that increases the size of the table will negatively impact the speed of those queries since they now have to read more data.

When you increase the size of a row, you decrease the number of rows per block. That would tend to increase the pressure on your buffer cache so you'd either be caching fewer rows from this table or you'd be aging out some other blocks faster. Either of those could lead to individual queries doing more physical I/O rather than logical I/O and thus running longer.

A VARCHAR2(1000) will only use whatever space is actually required to store a particular value. If some rows only need 100 bytes, Oracle would only allocate 100 bytes within the block. If other rows need 900 bytes, Oracle would allocate 900 bytes within the block.