dplyr (version 0.5.0)

explain: Explain details of a tbl.

Description

This is a generic function which gives more details about an object than print, and is more focussed on human readable output than str.

Usage

explain(x, ...)
show_query(x)

Arguments

x

An object to explain

...

Other parameters possibly used by generic

Databases

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.

Examples

if (require("RSQLite") && has_lahman("sqlite")) {

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()
}