Riaz Arbi created ARROW-16010:
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Summary: write_parquet alters <dttm> value
Key: ARROW-16010
URL: https://issues.apache.org/jira/browse/ARROW-16010
Project: Apache Arrow
Issue Type: Bug
Components: R
Affects Versions: 6.0.0
Environment: Ubuntu focal
R 4.1.1
RStudio 1.4.1772
Reporter: Riaz Arbi
When we write a dataframe column of type `<dttm>` to parquet using the arrow
package, subsequent reading in of the parquet file to dataframe returns a
slightly different value.
This behaviour does not replicate with columns of type `<double>`
Reprex:
```
# Create sample dataframe
n <- 1631494810.376999855041503906250000000000000000000000000000000000
df <- data.frame(x = "a",
n = n,
t = as.POSIXct(n, origin = "1970-01-01"))
# Write to disk
df %>% write_parquet("/tmp/tmp.parquet")
# Extract time-based cols
dft <- df %>%
filter(x == "a") %>%
pull(t) %>%
as.numeric
pqt <- read_parquet("/tmp/tmp.parquet") %>%
filter(x == "a") %>%
pull(t) %>%
as.numeric
dft == pqt
sprintf("%.54f",dft)
sprintf("%.54f",pqt)
# Extract numeric cols
dfn <- df %>%
filter(x == "a") %>%
pull(n) %>%
as.numeric
pqn <- read_parquet("/tmp/tmp.parquet") %>%
filter(x == "a") %>%
pull(n) %>%
as.numeric
dfn == pqn
sprintf("%.54f",dfn)
sprintf("%.54f",pqn)
```
The critical issue is that `dft == pqt` returns `FALSE` while `dfn == pqn`
returns TRUE.
Why is this a problem? We use `arrow` to store dataframes to disk. When we want
to update these parquet files, we first check whether any data has actually
changed and put in place tripwires to ensure that if a significant proportion
of the data has changed the pipeline fails and is flagged for manual review.
With the current behaviour, above, all of the dataframes that contain `<dttm>`
type columns are failing.
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