Is 10,000 Cells Big?

February 13, 2018
By

(This article was originally published at Statistics – Win-Vector Blog, and syndicated at StatsBlogs.)

Trick question: is a 10,000 cell numeric data.frame big or small?

In the era of "big data" 10,000 cells is minuscule. Such data could be fit on fewer than 1,000 punched cards (or less than half a box).


Punch card

The joking answer is: it is small when they are selling you the system, but can be considered unfairly large later.

Example

Let’s look at a few examples in R. First let’s set up our examples. A 10,000 row by one column data.frame (probably fairly close the common mental model of a 10,000 cell data.frame), and a 10,000 column by one row data.frame (frankly bit of an abuse, but data warehouse tables with millions of rows and 500 to 1,000 columns are not uncommon).

dTall <- as.data.frame(matrix(data = 0.0, 
                              nrow = 10000, 
                              ncol = 1))

dWide <- as.data.frame(matrix(data = 0.0, 
                              nrow = 1,
                              ncol = 10000))

For our example problem we will try to select (zero) rows based on a condition written against the first column.

Base R

For standard R working with either data.frame is not a problem.

system.time(nrow(dTall[dTall$V1>0, , drop = FALSE]))
##    user  system elapsed 
##       0       0       0
system.time(nrow(dWide[dWide$V1>0, , drop = FALSE]))
##    user  system elapsed 
##   0.060   0.004   0.064

dplyr

For dplyr the tall frame is no problem, but the wide frame is slow.

library("dplyr")
## 
## Attaching package: 'dplyr'

## The following objects are masked from 'package:stats':
## 
##     filter, lag

## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union
system.time(dTall %>% filter(V1>0) %>% tally())
##    user  system elapsed 
##   0.059   0.003   0.061
system.time(dWide %>% filter(V1>0) %>% tally())
##    user  system elapsed 
##   2.224   0.087   2.320

We will dig deeper into the dplyr timing on the wide table later.

Databases

Most databases don’t really like to work with a ridiculous number of columns.

RSQLite

RSQLite refuses to worm with the wide frame.

db <- DBI::dbConnect(RSQLite::SQLite(), 
                     ":memory:")
DBI::dbWriteTable(db, "dTall", dTall,
                  overwrite = TRUE,
                  temporary = TRUE)

DBI::dbWriteTable(db, "dWide", dWide,
                  overwrite = TRUE,
                  temporary = TRUE)
## Error in rsqlite_send_query(conn@ptr, statement): too many columns on dWide
DBI::dbDisconnect(db)

RPostgres

RPostgres refuses the wide frame, stating a hard limit of 1600 columns.

db <- DBI::dbConnect(RPostgres::Postgres(),
                     host = 'localhost',
                     port = 5432,
                     user = 'postgres',
                     password = 'pg')
DBI::dbWriteTable(db, "dTall", dTall,
                  overwrite = TRUE,
                  temporary = TRUE)

DBI::dbWriteTable(db, "dWide", dWide,
                  overwrite = TRUE,
                  temporary = TRUE)
## Error in result_create(conn@ptr, statement): Failed to fetch row: ERROR:  tables can have at most 1600 columns
DBI::dbDisconnect(db)

sparklyr

sparklyr fails, losing the cluster connection when attempting to write the wide frame.

spark <- sparklyr::spark_connect(version='2.2.0', 
                                 master = "local")
DBI::dbWriteTable(spark, "dTall", dTall,
                  temporary = TRUE)

DBI::dbWriteTable(db, "dWide", dWide,
                  temporary = TRUE)
## Error in connection_quote_identifier(conn@ptr, x): Invalid connection
sparklyr::spark_disconnect(spark)

Why I care

Some clients have run into intermittent issues on Spark at around 700 columns. One step of working around the issue was trying a range of sizes to try and figure out where the issue was and get a repeatable failure ( always an important step in debugging).

Extra: dplyr again at larger scale.

Let’s look a bit more closely at that dplyr run-time. We will try to get the nature of the column dependency by pushing the column count ever further up: to 100,000.

This is still less than a megabyte of data. It can fit on a 1986 era 1.44 MB floppy disk.


Floppy disk 300 dpi

dWide <- as.data.frame(matrix(data = 0.0, 
                              nrow = 1,
                              ncol = 100000))

dwt <- system.time(dWide %>% filter(V1>0) %>% tally())
print(dwt)
##    user  system elapsed 
## 251.441  28.067 283.060

Python

For comparison we can measure how long it would take to write the results out to disk, start up a Python interpreter, use Pandas do do the work, and then read the result back in to R.

start_pandas <- Sys.time()
feather::write_feather(dWide, "df.feather")
import pandas
import feather
df = feather.read_dataframe('df.feather')
print(type(df))
## <class 'pandas.core.frame.DataFrame'>
print(df.shape)
## (1, 100000)
df_filtered = df.query('V1>1')
feather.write_dataframe(df_filtered, 'dr.feather')
res <- feather::read_feather('dr.feather')
nrow(res)
## [1] 0
end_pandas <- Sys.time()
python_duration <- difftime(end_pandas, start_pandas, 
                            unit = "secs")
print(python_duration)
## Time difference of 21.47297 secs
ratio <- as.numeric(dwt['elapsed'])/as.numeric(python_duration)
print(ratio)
## [1] 13.18216

This is slow, but still 13.2 times faster than using dplyr.



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