Every additional index that you define on a table adds to the costs associated with your database.

在一个表上定义的每个额外索引都将增加您的数据库关联成本。

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Sometimes, you may find that additional index specifications can also be defined to improve performance.

有时候，您可能发现还可以通过定义附加的索引规范来提高性能。

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Additional index definitions that would improve performance of the queries considered by reducing the runtime of the queries.

额外的索引定义，它们通过减少查询的运行时间改进查询性能。

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Instead, in March 2013 it will launch an additional index called RPIJ, a new version of the RPI index but using the same methods as CPI for calculating average prices.

BBC: Q&A: Inflation changes

When a portfolio manager for an index fund performs additional management services, the fund is no longer passive.

FORBES: The Hidden Differences Between Index Funds

In these cases, the ETF becomes riskier than the index it tracks and a lack of commission may not be enough to justify the additional risk over buying a mutual fund that tracks the same index.

FORBES: Commission-Free ETFs Are Not Really Free