title: Getting different results when using the sparklyr and dplyr
body: <p>Just now i start learning the sparklyr package using the reference <a href="http://spark.rstudio.com/" rel="nofollow noreferrer">sparklyr</a>
   i did what was  written in the document. 
  when using the following code</p>
  
  <pre><code>delay &lt;- flights_tbl %&gt;% 
    group_by(tailnum) %&gt;%
    summarise(count = n(), dist = mean(distance), delay = mean(arr_delay)) %&gt;%
    filter(count &gt; 20, dist &lt; 2000, !is.na(delay)) %&gt;%
    collect
  
  Warning messages:
  1: Missing values are always removed in SQL.
  Use `AVG(x, na.rm = TRUE)` to silence this warning 
  2: Missing values are always removed in SQL.
  Use `AVG(x, na.rm = TRUE)` to silence this warning 
  
  &gt; delay
  # A tibble: 2,961 x 4
     tailnum count  dist  delay
     &lt;chr&gt;   &lt;dbl&gt; &lt;dbl&gt;  &lt;dbl&gt;
   1 N14228  111    1547  3.71 
   2 N24211  130    1330  7.70 
   3 N668DN   49.0  1028  2.62 
   4 N39463  107    1588  2.16 
   5 N516JB  288    1249 12.0  
   6 N829AS  230     228 17.2  
   7 N3ALAA   63.0  1078  3.59 
   8 N793JB  283    1529  4.72 
   9 N657JB  285    1286  5.03 
  10 N53441  102    1661  0.941
  # ... with 2,951 more rows
  </code></pre>
  
  <p>In the similar way i want apply the same operations on  <code>nycflights13::flights</code> dataset using <code>dplyr</code> package </p>
  
  <pre><code>nycflights13::flights %&gt;% 
      group_by(tailnum) %&gt;%
      summarise(count = n(), dist = mean(distance), delay = mean(arr_delay)) %&gt;%
       filter(count &gt; 20, dist &lt; 2000, !is.na(delay)) 
  
  # A tibble: 1,319 x 4
     tailnum count  dist   delay
     &lt;chr&gt;   &lt;int&gt; &lt;dbl&gt;   &lt;dbl&gt;
   1 N102UW     48   536   2.94 
   2 N103US     46   535 - 6.93 
   3 N105UW     45   525 - 0.267
   4 N107US     41   529 - 5.73 
   5 N108UW     60   534 - 1.25 
   6 N109UW     48   536 - 2.52 
   7 N110UW     40   535   2.80 
   8 N111US     30   536 - 0.467
   9 N11206    111  1414  12.7  
  10 N112US     38   535 - 0.947
  # ... with 1,309 more rows
  </code></pre>
  
  <p>My problem is why i am getting the different results ?</p>
  
  <p>As mention in the documentation  <code>dplyr</code> is the complete backend operations 
  for <code>sparklyr</code>.</p>
  
  <pre><code> &gt; sessionInfo()
  R version 3.4.0 (2017-04-21)
  Platform: i386-w64-mingw32/i386 (32-bit)
  Running under: Windows 7 (build 7601) Service Pack 1
  
  Matrix products: default
  
  locale:
  [1] LC_COLLATE=English_United States.1252 [2] LC_CTYPE=English_United States.1252   
  [3] LC_MONETARY=English_United States.1252 [4] LC_NUMERIC=C                          
  [5] LC_TIME=English_United States.1252    
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] bindrcpp_0.2   dplyr_0.7.4    sparklyr_0.7.0
  
  loaded via a namespace (and not attached):
   [1] DBI_0.7            readr_1.1.1        withr_2.1.1       
   [4] nycflights13_0.2.2 rprojroot_1.3-2    lattice_0.20-35   
   [7] foreign_0.8-69     pkgconfig_2.0.1    config_0.2        
  [10] utf8_1.1.3         compiler_3.4.0     stringr_1.3.0     
  [13] parallel_3.4.0     xtable_1.8-2       Rcpp_0.12.15      
  [16] cli_1.0.0          shiny_1.0.5        plyr_1.8.4        
  [19] httr_1.3.1         tools_3.4.0        openssl_1.0       
  [22] nlme_3.1-131.1     broom_0.4.3        R6_2.2.2          
  [25] dbplyr_1.2.1       bindr_0.1          purrr_0.2.4       
  [28] assertthat_0.2.0   curl_3.1           digest_0.6.15     
  [31] mime_0.5           stringi_1.1.6      rstudioapi_0.7    
  [34] reshape2_1.4.3     hms_0.4.1          backports_1.1.2   
  [37] htmltools_0.3.6    grid_3.4.0         glue_1.2.0        
  [40] httpuv_1.3.5       rlang_0.2.0        psych_1.7.8       
  [43] magrittr_1.5       rappdirs_0.3.1     lazyeval_0.2.1    
  [46] yaml_2.1.16        crayon_1.3.4       tidyr_0.8.0       
  [49] pillar_1.1.0       base64enc_0.1-3    mnormt_1.5-5      
  [52] jsonlite_1.5       tibble_1.4.2       Lahman_6.0-0     
  </code></pre>