What Is SQL?
Big Data and SQL: Apache Hive
Big Data and SQL: Spark SQL
The power of Spark SQL derives from several key facts: SQL analysts can now take advantage of Spark’s computation abilities by plugging into the Thrift Server or Spark’s SQL interface, whereas data engineers and scientists can use Spark SQL where appropriate in any data flow. This unifying API allows for data to be extracted with SQL, manipulated as a DataFrame, passed into one of Spark MLlibs’ large-scale machine learning algorithms, written out to another data source, and everything in between.
Note: Spark SQL is intended to operate as an online analytic processing (OLAP) database, not an online transaction processing (OLTP) database. This means that it is not intended to perform extremely low-latency queries. Even though support for in-place modifications is sure to be something that comes up in the future, it’s not something that is currently available.
Spark’s Relationship to Hive
Spark SQL has a great relationship with Hive because it can connect to Hive metastores. The Hive metastore is the way in which Hive maintains table information for use across sessions. With Spark SQL, you can connect to your Hive metastore (if you already have one) and access table metadata to reduce file listing when accessing information.
The Hive metastore
To connect to the Hive metastore, there are several properties that you’ll need. First, you need to set the Metastore version (spark.sql.hive.metastore.version) to correspond to the proper Hive metastore that you’re accessing. By default, this value is 1.2.1. You also need to set spark.sql.hive.metastore.jars if you’re going to change the way that the HiveMetastoreClient is initialized. Spark uses the default versions, but you can also specify Maven repositories or a classpath in the standard format for the Java Virtual Machine (JVM). In addition, you might need to supply proper class prefixes in order to communicate with different databases that store the Hive metastore. You’ll set these as shared prefixes that both Spark and Hive will share (spark.sql.hive.metastore.sharedPrefixes).
If you’re connecting to your own metastore, it’s worth checking the documentation for further updates and more information.
How to Run Spark SQL Queries
Spark SQL CLI
The Spark SQL CLI is a convenient tool with which you can make basic Spark SQL queries in local mode from the command line. Note that the Spark SQL CLI cannot communicate with the Thrift JDBC server. To start the Spark SQL CLI, run the following in the Spark directory:
./bin/spark-sql
You configure Hive by placing your hive-site.xml, core-site.xml, and hdfs-site.xml files in conf/. available options, you can run ./bin/spark-sql --help.
Spark’s Programmatic SQL Interface
In addition to setting up a server, you can also execute SQL in an ad hoc manner via any of Spark’s language APIs. You can do this via the method sql on the SparkSession object. This returns a DataFrame, as we will see later in this chapter.For example, in Python or Scala, we can run the following:
spark.sql("SELECT 1 + 1").show()
this will not be executed eagerly but lazily. This is an immensely powerful interface because there are some transformations that are much simpler to express in SQL code than in DataFrames.
You can express multiline queries quite simply by passing a multiline string into the function. For example, you could execute something like the following code in Python or Scala:
spark.sql("""SELECT user_id, department, first_name FROM professors
WHERE department IN
(SELECT name FROM department WHERE created_date >= '2016-01-01')""")
Even more powerful, you can completely interoperate between SQL and DataFrames, as you see fit.can create a DataFrame, manipulate it with SQL, and then manipulate it again as a DataFrame. It’s a powerful abstraction that you will likely find yourself using quite a bit.
// in Scala
spark.read.json("/data/flight-data/json/2015-summary.json")
.createOrReplaceTempView("some_sql_view") // DF => SQL
spark.sql("""
SELECT DEST_COUNTRY_NAME, sum(count)
FROM some_sql_view GROUP BY DEST_COUNTRY_NAME
""")
.where("DEST_COUNTRY_NAME like 'S%'").where("`sum(count)` > 10")
.count() // SQL => DF
SparkSQL Thrift JDBC/ODBC Server
Spark provides a Java Database Connectivity (JDBC) interface by which either you or a remote program connects to the Spark driver in order to execute Spark SQL queries. A common use case might be a for a business analyst to connect business intelligence software like Tableau to Spark. The Thrift JDBC/Open Database Connectivity (ODBC) server implemented here corresponds to the HiveServer2 in Hive 1.2.1. You can test the JDBC server with the beeline script that comes with either Spark or Hive 1.2.1.
To start the JDBC/ODBC server, run the following in the Spark directory:
./sbin/start-thriftserver.sh
This script accepts all bin/spark-submit command-line options. To see all available options for configuring this Thrift Server, run ./sbin/start-thriftserver.sh --help. By default, the server listens on localhost:10000. You can override this through environmental variables or system properties.
For environment configuration, use this:
export HIVE_SERVER2_THRIFT_PORT=<listening-port>
export HIVE_SERVER2_THRIFT_BIND_HOST=<listening-host>
./sbin/start-thriftserver.sh
--master <master-uri>
...
For system properties:
./sbin/start-thriftserver.sh
--hiveconf hive.server2.thrift.port=<listening-port>
--hiveconf hive.server2.thrift.bind.host=<listening-host>
--master <master-uri>
...
You can then test this connection by running the following commands:
./bin/beeline
beeline> !connect jdbc:hive2://localhost:10000
Beeline will ask you for a username and password. In nonsecure mode, simply type the username on your machine and a blank password. For secure mode, follow the instructions given in the beeline documentation.
Catalog
The highest level abstraction in Spark SQL is the Catalog. The Catalog is an abstraction for the storage of metadata about the data stored in your tables as well as other helpful things like databases, tables, functions, and views. The catalog is available in the org.apache.spark.sql.catalog.Catalog package and contains a number of helpful functions for doing things like listing tables, databases, and functions.
This chapter shows only the SQL being executed; thus, if you’re using the programmatic interface, keep in mind that you need to wrap everything in a spark.sql function call to execute the relevant code.
Tables
The core difference between tables and DataFrames is this: you define DataFrames in the scope of a programming language, whereas you define tables within a database. This means that when you create a table (assuming you never changed the database), it will belong to the default database. We discuss databases more fully later on in the chapter.
An important thing to note is that in Spark 2.X, tables always contain data. There is no notion of a temporary table, only a view, which does not contain data. This is important because if you go to drop a table, you can risk losing the data when doing so.
Spark-Managed Tables
One important note is the concept of managed versus unmanaged tables. Tables store two important pieces of information. The data within the tables as well as the data about the tables; that is, the metadata. You can have Spark manage the metadata for a set of files as well as for the data. When you define a table from files on disk, you are defining an unmanaged table. When you use saveAsTable on a DataFrame, you are creating a managed table for which Spark will track of all of the relevant information.
This will read your table and write it out to a new location in Spark format. You can see this reflected in the new explain plan. In the explain plan, you will also notice that this writes to the default Hive warehouse location. You can set this by setting the spark.sql.warehouse.dir configuration to the directory of your choosing when you create your SparkSession. By default Spark sets this to /user/hive/warehouse:
you can also see tables in a specific database by using the query show tables IN databaseName, where databaseName represents the name of the database that you want to query.
Creating Tables
You can create tables from a variety of sources. Something fairly unique to Spark is the capability of reusing the entire Data Source API within SQL. This means that you do not need to define a table and then load data into it; Spark lets you create one on the fly. You can even specify all sorts of sophisticated options when you read in a file.For example, here’s a simple way to read in the flight data we worked with in previous chapters:
CREATE TABLE flights (
DEST_COUNTRY_NAME STRING, ORIGIN_COUNTRY_NAME STRING, count
LONG)
USING JSON OPTIONS (path '/data/flight-data/json/2015-summary.json')
笔记：USING and STORED AS
The specification of the USING syntax in the previous example is of significant importance. If you do not specify the format, Spark will default to a Hive SerDe configuration. This has performance implications for future readers and writers because Hive SerDes are much slower than Spark’s native serialization. Hive users can also use the STORED AS syntax to specify that this should be a Hive table.
You can also add comments to certain columns in a table, which can help other developers understand the data in the
tables:
CREATE TABLE flights_csv (
DEST_COUNTRY_NAME STRING,
ORIGIN_COUNTRY_NAME STRING COMMENT "remember, the US will be most prevalent",
count LONG)
USING csv OPTIONS (header true, path '/data/flight-data/csv/2015-summary.csv')
It is possible to create a table from a query as well:
CREATE TABLE flights_from_select USING parquet AS SELECT * FROM flights
In addition, you can specify to create a table only if it does not currently exist:
Note
In this example, we are creating a Hive-compatible table because we did not explicitly specify the format via USING. We can also do the following:
CREATE TABLE IF NOT EXISTS flights_from_select
AS SELECT * FROM flights
Finally, you can control the layout of the data by writing out a partitioned dataset
CREATE TABLE partitioned_flights USING parquet PARTITIONED BY (DEST_COUNTRY_NAME)
AS SELECT DEST_COUNTRY_NAME, ORIGIN_COUNTRY_NAME, count FROM flights LIMIT 5
These tables will be available in Spark even through sessions; temporary tables do not currently exist in Spark. You must create a temporary view, which we demonstrate later in this chapter.
Creating External Tables
we create an unmanaged table. Spark will manage the table’s metadata; however, the files are not managed by Spark at all. You create this table by using the CREATE EXTERNAL TABLE statement.
CREATE EXTERNAL TABLE hive_flights (
DEST_COUNTRY_NAME STRING, ORIGIN_COUNTRY_NAME STRING, count LONG)
ROW FORMAT DELIMITED FIELDS TERMINATED BY ',' LOCATION '/data/flight-data-hive/'
You can also create an external table from a select clause
CREATE EXTERNAL TABLE hive_flights_2
ROW FORMAT DELIMITED FIELDS TERMINATED BY ','
LOCATION '/data/flight-data-hive/' AS SELECT * FROM flights
Inserting into Tables
Insertions follow the standard SQL syntax:
INSERT INTO flights_from_select
SELECT DEST_COUNTRY_NAME, ORIGIN_COUNTRY_NAME, count FROM flights LIMIT 20
You can optionally provide a partition specification if you want to write only into a certain partition.Note that a write will respect a partitioning scheme, as well (which may cause the above query to run quite slowly); however, it will add additional files only into the end partitions:
INSERT INTO partitioned_flights
PARTITION (DEST_COUNTRY_NAME="UNITED STATES")
SELECT count, ORIGIN_COUNTRY_NAME FROM flights
WHERE DEST_COUNTRY_NAME='UNITED STATES' LIMIT 12
Describing Table Metadata
DESCRIBE TABLE flights_csv
You can also see the partitioning scheme for the data by using the following (note, however, that this works only on partitioned tables):
SHOW PARTITIONS partitioned_flights
Refreshing Table Metadata
Maintaining table metadata is an important task to ensure that you’re reading from the most recent set of data. There are two commands to refresh table metadata. REFRESH TABLE refreshes all cached entries (essentially, files) associated with the table. If the table were previously cached, it would be cached lazily the next time it is scanned:
REFRESH table partitioned_flights
Another related command is REPAIR TABLE, which refreshes the partitions maintained in the catalog for that given table. This command’s focus is on collecting new partition information—an example might be writing out a new partition manually and the need to repair the table accordingly:
MSCK REPAIR TABLE partitioned_flights
Dropping Tables
You cannot delete tables: you can only “drop” them. You can drop a table by using the DROP keyword. If you drop a managed table (e.g., flights_csv), both the data and the table definition will be removed:
DROP TABLE flights_csv;
If you try to drop a table that does not exist, you will receive an error. To only delete a table if it already exists, use DROP TABLE IF EXISTS.
DROP TABLE IF EXISTS flights_csv;
Dropping unmanaged tables
If you are dropping an unmanaged table (e.g., hive_flights), no data will be removed but you will no longer be able to refer to this data by the table name.
Caching Tables
Just like DataFrames, you can cache and uncache tables. You simply specify which table you would like using the following syntax:
CACHE TABLE flights
Here’s how you uncache them:
UNCACHE TABLE FLIGHTS
Views
A view specifies a set of transformations on top of an existing table—basically just saved query plans, which can be convenient for organizing or reusing your query logic. Spark has several different notions of views. Views can be global, set to a database, or per session.
Creating Views
To an end user, views are displayed as tables, except rather than rewriting all of the data to a new location, they simply perform a transformation on the source data at query time. This might be a filter, select, or potentially an even larger GROUP BY or ROLLUP. For instance, in the following example, we create a view in which the destination is United States in order to see only those flights:
CREATE VIEW just_usa_view AS
SELECT * FROM flights WHERE dest_country_name = 'United States'
Like tables, you can create temporary views that are available only during the current session and are not registered to a database:
CREATE TEMP VIEW just_usa_view_temp AS
SELECT * FROM flights WHERE dest_country_name = 'United States'
Or, it can be a global temp view. Global temp views are resolved regardless of database and are viewable across the entire Spark application, but they are removed at the end of the session:
CREATE GLOBAL TEMP VIEW just_usa_global_view_temp AS
SELECT * FROM flights WHERE dest_country_name = 'United States'
SHOW TABLES
You can also specify that you would like to overwite a view if one already exists by using the keywords shown in the sample that follows. We can overwrite both temp views and regular views:
CREATE OR REPLACE TEMP VIEW just_usa_view_temp AS
SELECT * FROM flights WHERE dest_country_name = 'United States'
Now you can query this view just as if it were another table:
SELECT * FROM just_usa_view_temp
A view is effectively a transformation and Spark will perform it only at query time. This means that it will only apply that filter after you actually go to query the table (and not earlier). Effectively, views are equivalent to creating a new DataFrame from an existing DataFrame.
In fact, you can see this by comparing the query plans generated by Spark DataFrames and Spark SQL.In DataFrames, we would write the following:
val flights = spark.read.format("json")
.load("/data/flight-data/json/2015-summary.json")
val just_usa_df = flights.where("dest_country_name = 'United
States'")
just_usa_df.selectExpr("*").explain
In SQL, we would write (querying from our view) this:
EXPLAIN SELECT * FROM just_usa_view
Or, equivalently:
EXPLAIN SELECT * FROM flights WHERE dest_country_name = 'United States'
Due to this fact, you should feel comfortable in writing your logic either on DataFrames or SQL—whichever is most comfortable and maintainable for you.
Dropping Views
You can drop views in the same way that you drop tables; you simply specify that what you intend to drop is a view instead of a table. The main difference between dropping a view and dropping a table is that with a view, no underlying data is removed, only the view definition itself:
DROP VIEW IF EXISTS just_usa_view;
Databases
if you do not define one, Spark will use the default database. Any SQL statements that you run from within Spark (including DataFrame commands) execute within the context of a database. This means that if you change the database, any user-defined tables will remain in the previous database and will need to be queried differently.
SHOW DATABASES
CREATE DATABASE some_db
USE some_db
you can query different databases by using the correct prefix:
SELECT * FROM default.flights
You can see what database you’re currently using by running the following command:
SELECT current_database()
DROP DATABASE IF EXISTS some_db;
Select Statements
SELECT [ALL|DISTINCT] named_expression[, named_expression, ...]
FROM relation[, relation, ...]
[lateral_view[, lateral_view, ...]]
[WHERE boolean_expression]
[aggregation [HAVING boolean_expression]]
[ORDER BY sort_expressions]
[CLUSTER BY expressions]
[DISTRIBUTE BY expressions]
[SORT BY sort_expressions]
[WINDOW named_window[, WINDOW named_window, ...]]
[LIMIT num_rows]
named_expression:
: expression [AS alias]
relation:
| join_relation
| (table_name|query|relation) [sample] [AS alias]
: VALUES (expressions)[, (expressions), ...]
[AS (column_name[, column_name, ...])]
expressions:
: expression[, expression, ...]
sort_expressions:
: expression [ASC|DESC][, expression [ASC|DESC], ...]
case…when…then Statements
SELECT
CASE WHEN DEST_COUNTRY_NAME = 'UNITED STATES' THEN 1
WHEN DEST_COUNTRY_NAME = 'Egypt' THEN 0
ELSE -1 END
FROM partitioned_flights
Advanced Topics
Complex Types
- Structs
Structs are more akin to maps. They provide a way of creating or querying nested data in Spark. To create one, you simply need to wrap a set of columns (or expressions) in parentheses:
CREATE VIEW IF NOT EXISTS nested_data AS
SELECT (DEST_COUNTRY_NAME, ORIGIN_COUNTRY_NAME) as country, count FROM flights
Now, you can query this data to see what it looks like:
SELECT * FROM nested_data
You can even query individual columns within a struct—all you need to do is use dot syntax:
SELECT country.DEST_COUNTRY_NAME, count FROM nested_data
If you like, you can also select all the subvalues from a struct by using the struct’s name and select all of the subcolumns. Although these aren’t truly subcolumns, it does provide a simpler way to think about them because we can do everything that we like with them as if they were a column:
SELECT country.*, count FROM nested_data
- Lists
You can use the collect_list function, which creates a list of values. You can also use the function collect_set, which creates an array without duplicate values. These are both aggregation functions and therefore can be specified only in aggregations:
SELECT DEST_COUNTRY_NAME as new_name, collect_list(count) as flight_counts,
collect_set(ORIGIN_COUNTRY_NAME) as origin_set
FROM flights GROUP BY DEST_COUNTRY_NAME
You can, however, also create an array manually within a column, as shown here:
SELECT DEST_COUNTRY_NAME, ARRAY(1, 2, 3) FROM flights
You can also query lists by position by using a Python-like array query syntax:
SELECT DEST_COUNTRY_NAME as new_name, collect_list(count)[0]
FROM flights GROUP BY DEST_COUNTRY_NAME
You can also do things like convert an array back into rows. You do this by using the explode function. To demonstrate, let’s create a new view as our aggregation:
CREATE OR REPLACE TEMP VIEW flights_agg AS
SELECT DEST_COUNTRY_NAME, collect_list(count) as collected_counts
FROM flights GROUP BY DEST_COUNTRY_NAME
Now let’s explode the complex type to one row in our result for every value in the array. The DEST_COUNTRY_NAME will duplicate for every value in the array, performing the exact opposite of the original collect and returning us to the original DataFrame:
SELECT explode(collected_counts), DEST_COUNTRY_NAME FROM flights_agg
Functions
To see a list of functions in Spark SQL, you use the SHOW FUNCTIONS statement:
SHOW FUNCTIONS
You can also more specifically indicate whether you would like to see the system functions (i.e., those built into Spark) as well as user functions:
SHOW SYSTEM FUNCTIONS
User functions are those defined by you or someone else sharing your Spark environment. These are the same user-defined functions that we talked about in earlier chapters (we will discuss how to create them later on in this chapter):
SHOW USER FUNCTIONS
You can filter all SHOW commands by passing a string with wildcard (*) characters. Here, we can see all functions that begin with “s”:
SHOW FUNCTIONS "s*";
Optionally, you can include the LIKE keyword, although this is not necessary:
SHOW FUNCTIONS LIKE "collect*";
Even though listing functions is certainly useful, often you might want to know more about specific functions themselves. To do this, use the DESCRIBE keyword, which returns the documentation for a specific function.
User-defined functions
You can also register functions through the Hive CREATE TEMPORARY FUNCTION syntax.
Subqueries
In Spark, there are two fundamental subqueries. Correlated subqueries use some information from the outer scope of the query in order to supplement information in the subquery. Uncorrelated subqueries include no information from the outer scope. Each of these queries can return one (scalar subquery) or more values. Spark also includes support for predicate subqueries, which allow for filtering based on values.
Uncorrelated predicate subqueries
SELECT * FROM flights
WHERE origin_country_name IN (SELECT dest_country_name FROM flights
GROUP BY dest_country_name ORDER BY sum(count) DESC LIMIT 5)
This query is uncorrelated because it does not include any information from the outer scope of the query. It’s a query that you can run on its own.
Correlated predicate subqueries
Correlated predicate subqueries allow you to use information from the outer scope in your inner query.For example, if you want to see whether you have a flight that will take you back from your destination country, you could do so by checking whether there is a flight that has the destination country as an origin and a flight that had the origin country as a destination:
SELECT * FROM flights f1
WHERE EXISTS (SELECT 1 FROM flights f2
WHERE f1.dest_country_name = f2.origin_country_name)
AND EXISTS (SELECT 1 FROM flights f2
WHERE f2.dest_country_name = f1.origin_country_name)
EXISTS just checks for some existence in the subquery and returns true if there is a value. You can flip this by placing the NOT operator in front of it. This would be equivalent to finding a flight to a destination from which you won’t be able to return!
Miscellaneous Features
Configurations
You can set these either at application initialization or over the course of application execution (like we have seen with shuffle partitions throughout this book).
Configurations表格
Setting Configuration Values in SQL
Naturally, you can only set Spark SQL configurations that way, but here’s how you can set shuffle partitions:
SET spark.sql.shuffle.partitions=20