parBatchByIndex {ChemmineR} | R Documentation

## Parallel Batch By Index

### Description

Takes an index set, breaks it into batches and runs the given function on each batch in parallel using the given cluster. See `batchByIndex` for the non-parallel version.

When doing a select were the condition is a large number of ids it is not always possible to include them in a single SQL statement. This function will break the list of ids into chunks and allow the indexProcessor to deal with just a small number of ids.

### Usage

    parBatchByIndex(allIndices, indexProcessor, reduce, cl, batchSize = 1e+05)

### Arguments

`allIndices` | A vector of values that will be broken into batches and passed as an argument to the `indexProcessor` function.
`indexProcessor` | A function that takes one batch if indices. It is called once for each batch, possibly in parallel. The return value of this function is collected into a list and passed to the `reduce` function after all jobs have finished.
`reduce` | This function is run after all jobs have finished. It is called with a list of return values from the `indexProcessor` function runs. The order of batchs is maintained. The return value of the `reduce` function is then returned. The idea is that this function merges all the results together into one result.
`cl` | A SNOW cluster to run jobs on.
`batchSize` | The size of each batch. The last batch may be smaller than this value.

### Value

The return value of the `reduce` function is returned.

### Author(s)

Kevin Horan

### See Also

`batchByIndex`

### Examples

    	## Not run:

    		cl = makeCluster(2) # create a SNOW cluster

    		#function to run a query for each batch of indexes
    		job = function(indexBatch)
    				dbGetQuery(dbConnection, paste("SELECT weight FROM table WHERE id IN (",paste(indexBatch,collapse=","),")"))

    		# function to combine all the results, in this case by summing them up
    		reduce = function(results) sum(unlist(results))

    		indices = 1:10000

    		#run queries in parallel and then sum the results
    		totalWeight = parBatchByIndex(indices,job,reduce,cl, 1000)

    ## End(Not run)

[Package ChemmineR version 3.39.2 Index]