Oracle vs SQL Server

Platform dependency

SQL Server is only operable on the Windows platform, a major limitation for it to be an enterprise solution. Oracle is available on multiple platforms such as Windows, all flavors of Unix from vendors such as IBM, Sun, Digital, HP, Sequent, etc. and VAX-VMS, as well as MVS. The multi-platform nature of Oracle makes it a true enterprise solution

Locking and concurrency

SQL Server has no multi-version consistency model, which means that "writers block readers and readers block writers" to ensure data integrity. In contrast, with Oracle, the rule is "readers don't block writers and writers don't block readers." This is possible without compromising data integrity because Oracle will dynamically re-create a read-consistent image for a reader of any requested data that has been changed but not yet committed. In other words, the reader will see the data as it was before the writer began changing it (until the writer commits). SQL Server's locking scheme is much simpler (less mature) and will result in a lot of delays/waits in a heavy OLTP environment.

Also, SQL Server will escalate row locks to page level locks when too many rows on a page are locked. This locks rows which are uninvolved in any updates for no good reason.

Performance and tuning

Object types

Here are some object types missing in SQL Server that exist in Oracle.

PL/SQL versus T-SQL

Clustering technology

In clustering technology, Oracle is light years ahead, since SQL Server has nothing like Oracle Parallel Server/RAC -- two instances acting on the same data in active-active configurations. And with the new version of Parallel Server in Oracle 9i, renamed as the Oracle Real Application Clusters, there is diskless contention

Other Major Differences:

• In SQL Server, the DBA has no "real" control over sorting and cache memory allocation. The memory allocation is decided only globally in the server properties memory folder, and that applies for ALL memory and not CACHING, SORTING, etc
• In SQL Server, no range partitioning of large tables and indexes. In Oracle, a large 100 GB table can be seamlessly partitioned at the database level into range partitions. For example, an invoice table can be partitioned into monthly partitions. Such partitioned tables and partitioned indexes give performance and maintenance benefits and are transparent to the application.
• There is no partitioning, no bitmap indexes, no reverse key indexes, no function-based indexes, no star query optimization in SQL Server.