r - 从 JDBC 连接读取数据时如何使用谓词？

Question

默认情况下，spark_read_jdbc() 将整个数据库表读取到 Spark 中。我使用以下语法来创建这些连接。

library(sparklyr)
library(dplyr)

config <- spark_config()
config$`sparklyr.shell.driver-class-path` <- "mysql-connector-java-5.1.43/mysql-connector-java-5.1.43-bin.jar"

sc <- spark_connect(master         = "local",
                    version        = "1.6.0",
                    hadoop_version = 2.4,
                    config         = config)

db_tbl <- sc %>%
  spark_read_jdbc(sc      = .,
                  name    = "table_name",  
                  options = list(url      = "jdbc:mysql://localhost:3306/schema_name",
                                 user     = "root",
                                 password = "password",
                                 dbtable  = "table_name"))

但是，我现在遇到了这样的情况:我在 MySQL 数据库中有一个表，并且我只想将该表的一个子集读入 Spark。

如何让 spark_read_jdbc 接受谓词？我尝试将谓词添加到选项列表但没有成功，

db_tbl <- sc %>%
  spark_read_jdbc(sc      = .,
                  name    = "table_name",  
                  options = list(url      = "jdbc:mysql://localhost:3306/schema_name",
                                 user       = "root",
                                 password   = "password",
                                 dbtable    = "table_name",
                                 predicates = "field > 1"))

Answer 1

您可以用查询替换dbtable:

db_tbl <- sc %>%
  spark_read_jdbc(sc      = .,
              name    = "table_name",  
              options = list(url      = "jdbc:mysql://localhost:3306/schema_name",
                             user     = "root",
                             password = "password",
                             dbtable  = "(SELECT * FROM table_name WHERE field > 1) as my_query"))

但对于像这样的简单条件，Spark 应该在您过滤时自动推送它:

db_tbl %>% filter(field > 1)

只需确保设置:

memory = FALSE

在spark_read_jdbc中。