This is a generic function which gives more details about an object than
print()
, and is more focused on human readable output than
str()
.
explain(x, ...) show_query(x, ...)
x | An object to explain |
---|---|
... | Other parameters possibly used by generic |
The first argument, invisibly.
Explaining a tbl_sql
will run the SQL EXPLAIN
command which
will describe the query plan. This requires a little bit of knowledge about
how EXPLAIN
works for your database, but is very useful for
diagnosing performance problems.
# \donttest{ if (require("dbplyr")) { lahman_s <- lahman_sqlite() batting <- tbl(lahman_s, "Batting") batting %>% show_query() batting %>% explain() # The batting database has indices on all ID variables: # SQLite automatically picks the most restrictive index batting %>% filter(lgID == "NL" & yearID == 2000L) %>% explain() # OR's will use multiple indexes batting %>% filter(lgID == "NL" | yearID == 2000) %>% explain() # Joins will use indexes in both tables teams <- tbl(lahman_s, "Teams") batting %>% left_join(teams, c("yearID", "teamID")) %>% explain() }#>#> Warning: there is no package called ‘dbplyr’# }