urn the number of keys in the current db  </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ExpireCommand.html">EXPIRE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;">  set a time to live in seconds on a key  </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ExpireCommand.html">PERSIST</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; paddin