SQLDataFrame: Lazy representation of SQL database in DataFrame metaphor

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last edit: 10/7/2019


SQL database are very commonly used in the storage of very large genomic data resources. Many useful tools, such as DBI, dbplyr have provided convenient interfaces for R users to check and manipulate the data. These tools represent the SQL tables in tidy formats and support lazy and quick aggregation operations (e.g, *_join, union, etc.) for tables from same resources. Cross database aggregation is also supported when opted (using copy=TRUE) but become very expensive due to the internal copying process of a whole table into the other connection. Use of advanced functions often involves specialized SQL knowledge which brings challenges for common R users. The interoperability of existing bioinformatics tools are suboptimal, e.g., the SummarizedExperiment container for representation of sequencing or genotyping experiments that many modern bioinformatics pipelines are based.

The SQLDataFrame package was developed using familiar DataFrame-like paradigm and lazily represents the very large dataset from different SQL databases, such as SQLite and MySQL. The DataFrame-like interface provides familiarity for common R users in easy data manipulations such as square bracket subsetting, rbinding, etc. For modern R users, it also recognizes the tidy data analysis and dplyr grammar by supporting %>%, select, filter, mutate, etc. More importantly, database type-specific strategies were implemented in SQLDataFrame to efficiently handle the cross-database operations without incurring any internally expensive processes (especially for database with write permission). Some previously difficult data operations are made quick and easy in R, such as cross-database ID matching and conversion, variant annotation extraction, etc. The scalability and interoperability of SQLDataFrame are expected to significantly promote the handling of very large genomic data resources and facilitating the overall bioinformatics analysis.

Currently SQLDataFrame supports the DBI backend of SQLite, MySQL and Google BigQuery, which are most commonly used SQL-based databases. In the future or upon feature request, we would implement this package so that users could choose to use different database backend for SQLDataFrame representation.

Here is a list of commonly used backends (bolded are already supported!):


  1. Download the package.
if (!requireNamespace("BiocManager", quietly = TRUE))

The development version is also available to download from Github.

  1. Load the package(s) into R session.

SQLDataFrame class

SQLDataFrame constructor

There are two ways to construct a SQLDataFrame object:

1) Provide an argument of conn, dbtable and dbkey. conn is a valid DBIConnection from SQLite or MySQL; dbtable specifies the database table name that is going to be represented as SQLDataFrame object. If only one table is available in the specified database name, this argument could be left blank. The dbkey argument is used to specify the column name in the table which could uniquely identify all the data observations (rows).

2) Provide dbtable, dbkey as specified above, and credentials to build valid DBIConnections. for SQLite, the credential argument includes dbname. For MySQL, the credential arguments are host, user, password. Additional to the credentials, users must provide the type argument to specify the SQL database type. Supported types are "SQLite" and "MySQL". If not specified, "SQLite" is used by default. Supported database tables could be on-disk or remote on the web or cloud.

dbfile <- system.file("extdata/test.db", package = "SQLDataFrame")
conn <- DBI::dbConnect(DBI::dbDriver("SQLite"), dbname = dbfile)
obj <- SQLDataFrame(conn = conn, dbtable = "state",
                    dbkey = "state")

construction from database credentials:

obj1 <- SQLDataFrame(dbname = dbfile, type = "SQLite",
                     dbtable = "state", dbkey = "state")
all.equal(obj, obj1)

Note that after reading the database table into SQLDataFrame, the key columns will be kept as fixed columns showing on the left hand side, with | separating key column(s) with the other columns. The ncol, colnames, and corresponding column subsetting will only apply to the non-key-columns.


Slot & accessors

To make the SQLDataFrame object as light and compact as possible, there are only 5 slots contained in the object: tblData, dbkey, dbnrows, dbconcatKey, indexes. Metadata information could be returned through these 5 slots using slot accessors or other utility functions.


Besides, many useful common methods are defined on SQLDataFrame object to make it a more DataFrame-like data structure. e.g., we can use dimnames() to return the row/colnames of the data. It returns an unnamed list, with the first element being rownames which is always NULL, and 2nd element being colnames (could also use colnames() method). dim() method is defined to return the dimension of the database table, which enables the nrow()/ncol() to extract a specific dimension. length() method is also defined which works same as ncol().
Note that the rownames(SQLDataFrame) would always be NULL as rownames are not supported in SQLDataFrame. However, ROWNAMES(obj) was implemented for the [ subsetting with characters.


NOTE that the dbtable() accessor only works for a SQLDataFrame object that the lazy tbl carried in tblData slot corresponds to a single database. If the SQLDataFrame was generated from rbind, union or *_join, call saveSQLDataFrame() to save the lazy tbl to disk so that dbtable() will be activated.

aa <- rbind(obj[1:5, ], obj[6:10, ])
dbtable(aa)  ## message
bb <- saveSQLDataFrame(aa, dbname = tempfile(fileext=".db"),
                       dbtable = "aa")


We could also construct a SQLDataFrame object directly from a file name. The makeSQLDataFrame function takes input of character value of file name for common text files (.csv, .txt, etc.), write into database tables, and open as SQLDataFrame object. Users could provide values for the dbname and dbtable argument. If NULL, default value for dbname would be a temporary database file, and dbtable would be the basename(filename) without extension.

NOTE that the input file must have one or multiple columns that could uniquely identify each observation (row) to be used the dbkey() for SQLDataFrame. Also the file must be rectangular, i.e., rownames are not accepted. But users could save rownames as a separate column.

mtc <- tibble::rownames_to_column(mtcars)[,1:6]
filename <- file.path(tempdir(), "mtc.csv")
write.csv(mtc, file= filename, row.names = FALSE)
aa <- makeSQLDataFrame(filename, dbkey = "rowname", sep = ",",
                       overwrite = TRUE)


With all the methods ([ subsetting, rbind, *_join, etc.,) provided in the next section, the SQLDataFrame always work like a lazy representation until users explicitly call the saveSQLDataFrame function for realization. saveSQLDataFrame write the lazy tbl carried in tblData slot into an on-disk database table, and re-open the SQLDataFrame object from the new path.

It's also recommended that users call saveSQLDataFrame frequently to avoid too many lazy layers which slows down the data processing.

obj1 <- saveSQLDataFrame(obj, dbname = tempfile(fileext = ".db"),
                        dbtable = "obj_copy")

SQLDataFrame methods

[[ subsetting

[[,SQLDataFrame Behaves similarly to [[,DataFrame and returns a realized vector of values from a single column. $,SQLDataFrame is also defined to conveniently extract column values.


We can also get the key column values using character extraction.


[ subsetting

SQLDataFrame instances can be subsetted in a similar way of DataFrame following the usual R conventions, with numeric, character or logical vectors; logical vectors are recycled to the appropriate length.

NOTE, use drop=FALSE explicitly for single column subsetting if you want to return a SQLDataFrame object, otherwise, the default drop=TRUE would always return a realized value for that column.

obj[1:3, 1:2]
obj[c(TRUE, FALSE), c(TRUE, FALSE), drop=FALSE]
obj[1:3, "population", drop=FALSE]
obj[, "population"]  ## realized column value

Subsetting with character vector works for the SQLDataFrame objects. With composite keys, users need to concatenate the key values by : for row subsetting (See the vignette for internal implementation for more details).

rnms <- ROWNAMES(obj)
obj[c("Alabama", "Colorado"), ]
obj1 <- SQLDataFrame(conn = conn, dbtable = "state",
                     dbkey = c("region", "population"))
rnms <- ROWNAMES(obj1)
obj1[c("South:3615.0", "West:365.0"), ]

List style subsetting is also allowed to extract certain columns from the SQLDataFrame object which returns SQLDataFrame by default.


filter & mutate

We have also enabled the S3 methods of filter and mutate from dplyr package, so that users could have the convenience in filtering data observations and adding new columns.

obj1 %>% filter(division == "South Atlantic" & size == "medium")
obj1 %>% mutate(p1 = population/10, s1 = size)

union & rbind

To be consistent with DataFrame, union and rbind methods were implemented for SQLDataFrame, where union returns the SQLDataFrame sorted by the dbkey(obj), and rbind keeps the original orders of input objects.

dbfile1 <- system.file("extdata/test.db", package = "SQLDataFrame")
con1 <- DBI::dbConnect(dbDriver("SQLite"), dbname = dbfile1)
dbfile2 <- system.file("extdata/test1.db", package = "SQLDataFrame")
con2 <- DBI::dbConnect(dbDriver("SQLite"), dbname = dbfile2)
ss1 <- SQLDataFrame(conn = con1, dbtable = "state",
                    dbkey = c("state"))
ss2 <- SQLDataFrame(conn = con2, dbtable = "state1",
                    dbkey = c("state"))
ss11 <- ss1[sample(5), ]
ss21 <- ss2[sample(10, 5), ]
obj1 <- union(ss11, ss21) 
obj1  ## reordered by the "dbkey()"
obj2 <- rbind(ss11, ss21) 
obj2  ## keeping the original order by updating the row index

*_join methods

The *_join family methods was implemented for SQLDataFrame objects, including the left_join, inner_join, semi_join and anti_join, which provides the capability of merging database files from different sources.

ss12 <- ss1[1:10, 1:2]
ss22 <- ss2[6:15, 3:4]
left_join(ss12, ss22)
inner_join(ss12, ss22)
semi_join(ss12, ss22)
anti_join(ss12, ss22)

Support of MySQL database data

SQLDataFrame now supports the MySQL database tables through RMySQL, for local MySQL servers, or remote ones on the web or cloud. The SQLDataFrame construction, *_join functions, union, rbind, and saving are all supported. Aggregation operations are supported for same or cross MySQL databases. Details please see the function documentations.

Here I'll show a simple use case for MySQL tables from ensembl.

ensbConn <- dbConnect(dbDriver("MySQL"),
                        user = "genome",
                        dbname = "xenTro9")
enssdf <- SQLDataFrame(conn = ensbConn,
                       dbtable = "xenoRefGene",
                       dbkey = c("name", "txStart"))
enssdf1 <- enssdf[1:20, 1:2]
enssdf2 <- enssdf[11:30,3:4]
res <- left_join(enssdf1, enssdf2)

Support of Google BigQuery

SQLDataFrame has just added support for Google BigQuery tables. Construction and queries using [ and filter are supported!

"Authentication and authorization" will be needed when using bigrquery. Check here for more details.

Also note that, the support of BigQuery tables has implemented specialized strategy for efficient data representation. The dbkey() is assigned by default as SurrogateKey, and dbkey argument will be ignored during construction.

bigrquery::bq_auth()  ## use this to authorize bigrquery in the
                      ## browser.
bqConn <- DBI::dbConnect(dbDriver("bigquery"),
                      project = "bigquery-public-data",
                      dataset = "human_variant_annotation",
                      billing = "") ## if not previous provided
                                    ## authorization, must specify a
                                    ## project name that was already
                                    ## linked with Google Cloud with
                                    ## billing info.
sdf <- SQLDataFrame(conn = bqConn, dbtable = "ncbi_clinvar_hg38_20180701")
sdf[1:5, 1:5]
sdf %>% select(GENEINFO)
sdf %>% filter(GENEINFO == "PYGL:5836")
sdf %>% filter(reference_name == "21")



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SQLDataFrame documentation built on Nov. 29, 2020, 2:01 a.m.