db2env: Load database into environment

Description Usage Arguments Details Value Author(s) See Also Examples

View source: R/filehash.R

Description

Load entire database into an environment

Usage

1
2
3
4
5
6
7
8
db2env(db)
dbLoad(db, ...)
dbLazyLoad(db, ...)

## S4 method for signature 'filehash'
dbLoad(db, env = parent.frame(2), keys = NULL, ...)
## S4 method for signature 'filehash'
dbLazyLoad(db, env = parent.frame(2), keys = NULL, ...)

Arguments

db

database object

env

an environment

keys

character vector of database keys to load

...

other arguments passed to methods

Details

db2env loads the entire database db into an environment via calls to makeActiveBinding. Therefore, the data themselves are not stored in the environment, but a function pointing to the data in the database is stored. When an element of the environment is accessed, the function is called to retrieve the data from the database. If the data in the database is changed, the changes will be reflected in the environment.

dbLoad loads objects in the database directly into the environment specified, like load does except with active bindings. dbLoad takes a second argument env, which is an environment, and the default for env is parent.frame().

The use of makeActiveBinding in db2env and dbLoad allows for potentially large databases to, at least conceptually, be used in R, as long as you don't need simultaneous access to all of the elements in the database.

With dbLazyLoad database objects are "lazy-loaded" into the environment. Promises to load the objects are created in the environment specified by env. Upon first access, those objects are copied into the environment and will from then on reside in memory. Changes to the database will not be reflected in the object residing in the environment after first access. Conversely, changes to the object in the environment will not be reflected in the database. This type of loading is useful for read-only databases.

Value

For db2env, an environment is returned, the elements of which are the keys of the database. For dbLoad and dbLazyLoad, a character vector is returned (invisibly) containing the keys associated with the values loaded into the environment.

Author(s)

Roger D. Peng

See Also

dbInit and filehash-class

Examples

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
dbCreate("myDB")
db <- dbInit("myDB")
dbInsert(db, "a", rnorm(100))
dbInsert(db, "b", 1:10)

env <- db2env(db)
ls(env)  ## "a", "b"
print(env$b)
mean(env$a)
env$a <- rnorm(100)
mean(env$a)

env$b[1:5] <- 5:1
print(env$b)

env <- new.env()
dbLoad(db, env)
ls(env)

env <- new.env()
dbLazyLoad(db, env)
ls(env)

as(db, "list")

rdpeng/filehash documentation built on May 24, 2017, 9:46 p.m.