When considering a link purchase or exchange, check the cache date of the page where your link will be located in Google.

当考虑一个购买或交换的链接时，请检查链接页面在google里的缓存的日期。

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If your table statistics are up to date, you can quickly and inexpensively check the cardinality of the columns in your existing table by issuing the following statement.

如果表的统计信息是最新的，那么就可以通过如下语句快速检查现有表内列的基数。

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The browser can safely not check for new versions until the expire date has passed. Setting this up in Apache is easy.

这个Expires HTTP头包含页面过期的时间，这样就可以避免浏览器在页面过期时间之前发送版本检查的请求。

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