explain: Explain details of a tbl

In olascodgreat/samife: A Grammar of Data Manipulation

Description

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

Usage

explain(x, ...)

show_query(x, ...)

Arguments

x	An object to explain
...	Other parameters possibly used by generic

Value

The first argument, invisibly.

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("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()
}

olascodgreat/samife documentation built on May 13, 2019, 6:11 p.m.