BRUSSELS - EU regulators are optimistic about achieving a satisfactory result from their review of U.S. software company Oracle Corp's (ORCL.O) plan to buy Sun Microsystems Inc (JAVA.O) despite competition concerns, Europe's antitrust chief said.

European Competition Commissioner Neelie Kroes's comments on Wednesday came a day before a hearing that could help regulators decide whether to clear or block the $7 billion deal, which has already been approved by the U.S. Department of Justice.

I am still optimistic that we can reach a satisfactory outcome that will ensure that there is no adverse impact on effective competition in the European market, Kroes told a news conference.

The European Commission has objected to the deal, citing possible competitive constraints on Sun's MySQL database after the takeover.

We are mainly focusing on the substantial reduction of competition in the market for the database which would arise precisely because of the acquisition by Oracle, Nadia Calvino, deputy director general for mergers at the commission, told an industry conference.

Oracle, Sun, competitors and interested parties will have the chance to argue their case in front of officials from the European Commission and European national competition regulators at the two-day hearing.

Representatives of Sweden's Ericsson (ERICb.ST); Britain's Vodafone Group Plc (VOD.L); the British Atomic Weapons Agency; the British National Health Service; Sabre Holdings Corp [TSG.UL] of the United States; Spain's Banco Bilbao Vizcaya Argentaria SA (BBVA.MC); UK User Group, which represents users of Oracle products; and Carnegie Mellon University will attend the hearing on Oracle's behalf, a person familiar with the matter told Reuters.

People familiar with the situation had told Reuters on Tuesday that U.S. software company Microsoft Corp (MSFT.O), Germany's SAP AG (SAPG.DE) and Monty Widenius, the creator of MySQL, would be present.

They said Oracle President Safra Catz and a representative from the Department of Justice were also due to attend.

Oracle has until Dec. 14 to submit remedies to the Commission. MySQL database, which competes mainly with Microsoft's SQL Server, is used to run popular websites operated by companies such as Google Inc (GOOG.O) and Amazon.com Inc (AMZN.O) and social networking site Facebook. (Editing by David Holmes and Gerald E. McCormick)

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