druid/docs/content/querying/sql.md

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SQL

Built-in SQL is an experimental feature. The API described here is subject to change.

Druid SQL is a built-in SQL layer and an alternative to Druid's native JSON-based query language, and is powered by a parser and planner based on Apache Calcite. Druid SQL translates SQL into native Druid queries on the query broker (the first node you query), which are then passed down to data nodes as native Druid queries. Other than the (slight) overhead of translating SQL on the broker, there isn't an additional performance penalty versus native queries.

To enable Druid SQL, make sure you have set druid.sql.enable = true either in your common.runtime.properties or your broker's runtime.properties.

Query syntax

Each Druid datasource appears as a table in the "druid" schema. This is also the default schema, so Druid datasources can be referenced as either druid.dataSourceName or simply dataSourceName.

Identifiers like datasource and column names can optionally be quoted using double quotes. To escape a double quote inside an identifier, use another double quote, like "My ""very own"" identifier". All identifiers are case-sensitive and no implicit case conversions are performed.

Literal strings should be quoted with single quotes, like 'foo'. Literal strings with Unicode escapes can be written like U&'fo\00F6', where character codes in hex are prefixed by a backslash. Literal numbers can be written in forms like 100 (denoting an integer), 100.0 (denoting a floating point value), or 1.0e5 (scientific notation). Literal timestamps can be written like TIMESTAMP '2000-01-01 00:00:00'. Literal intervals, used for time arithmetic, can be written like INTERVAL '1' HOUR, INTERVAL '1 02:03' DAY TO MINUTE, INTERVAL '1-2' YEAR TO MONTH, and so on.

Druid SQL supports SELECT queries with the following structure:

[ EXPLAIN PLAN FOR ]
[ WITH tableName [ ( column1, column2, ... ) ] AS ( query ) ]
SELECT [ ALL | DISTINCT ] { * | exprs }
FROM table
[ WHERE expr ]
[ GROUP BY exprs ]
[ HAVING expr ]
[ ORDER BY expr [ ASC | DESC ], expr [ ASC | DESC ], ... ]
[ LIMIT limit ]

The FROM clause refers to either a Druid datasource, like druid.foo, an INFORMATION_SCHEMA table, a subquery, or a common-table-expression provided in the WITH clause. If the FROM clause references a subquery or a common-table-expression, and both levels of queries are aggregations and they cannot be combined into a single level of aggregation, the overall query will be executed as a nested GroupBy.

The WHERE clause refers to columns in the FROM table, and will be translated to native filters. The WHERE clause can also reference a subquery, like WHERE col1 IN (SELECT foo FROM ...). Queries like this are executed as semi-joins, described below.

The GROUP BY clause refers to columns in the FROM table. Using GROUP BY, DISTINCT, or any aggregation functions will trigger an aggregation query using one of Druid's three native aggregation query types. GROUP BY can refer to an expression or a select clause ordinal position (like GROUP BY 2 to group by the second selected column).

The HAVING clause refers to columns that are present after execution of GROUP BY. It can be used to filter on either grouping expressions or aggregated values. It can only be used together with GROUP BY.

The ORDER BY clause refers to columns that are present after execution of GROUP BY. It can be used to order the results based on either grouping expressions or aggregated values. ORDER BY can refer to an expression or a select clause ordinal position (like ORDER BY 2 to order by the second selected column). For non-aggregation queries, ORDER BY can only order by the __time column. For aggregation queries, ORDER BY can order by any column.

The LIMIT clause can be used to limit the number of rows returned. It can be used with any query type. It is pushed down to data nodes for queries that run with the native TopN query type, but not the native GroupBy query type. Future versions of Druid will support pushing down limits using the native GroupBy query type as well. If you notice that adding a limit doesn't change performance very much, then it's likely that Druid didn't push down the limit for your query.

Add "EXPLAIN PLAN FOR" to the beginning of any query to see how it would be run as a native Druid query. In this case, the query will not actually be executed.

Aggregation functions

Aggregation functions can appear in the SELECT clause of any query. Any aggregator can be filtered using syntax like AGG(expr) FILTER(WHERE whereExpr). Filtered aggregators will only aggregate rows that match their filter. It's possible for two aggregators in the same SQL query to have different filters.

Only the COUNT aggregation can accept DISTINCT.

Function	Notes
COUNT(*)	Counts the number of rows.
COUNT(DISTINCT expr)	Counts distinct values of expr, which can be string, numeric, or hyperUnique. By default this is approximate, using a variant of HyperLogLog. To get exact counts set "useApproximateCountDistinct" to "false". If you do this, expr must be string or numeric, since exact counts are not possible using hyperUnique columns. See also APPROX_COUNT_DISTINCT(expr). In exact mode, only one distinct count per query is permitted.
SUM(expr)	Sums numbers.
MIN(expr)	Takes the minimum of numbers.
MAX(expr)	Takes the maximum of numbers.
AVG(expr)	Averages numbers.
APPROX_COUNT_DISTINCT(expr)	Counts distinct values of expr, which can be a regular column or a hyperUnique column. This is always approximate, regardless of the value of "useApproximateCountDistinct". See also COUNT(DISTINCT expr).
APPROX_QUANTILE(expr, probability, [resolution])	Computes approximate quantiles on numeric or approxHistogram exprs. The "probability" should be between 0 and 1 (exclusive). The "resolution" is the number of centroids to use for the computation. Higher resolutions will give more precise results but also have higher overhead. If not provided, the default resolution is 50. The approximate histogram extension must be loaded to use this function.

Numeric functions

Numeric functions will return 64 bit integers or 64 bit floats, depending on their inputs.

Function	Notes
ABS(expr)	Absolute value.
CEIL(expr)	Ceiling.
EXP(expr)	e to the power of expr.
FLOOR(expr)	Floor.
LN(expr)	Logarithm (base e).
LOG10(expr)	Logarithm (base 10).
POWER(expr, power)	expr to a power.
SQRT(expr)	Square root.
TRUNCATE(expr[, digits])	Truncate expr to a specific number of decimal digits. If digits is negative, then this truncates that many places to the left of the decimal point. Digits defaults to zero if not specified.
TRUNC(expr[, digits])	Synonym for TRUNCATE.
x + y	Addition.
x - y	Subtraction.
x * y	Multiplication.
x / y	Division.
MOD(x, y)	Modulo (remainder of x divided by y).

String functions

String functions accept strings, and return a type appropriate to the function.

Function	Notes
x || y	Concat strings x and y.
LENGTH(expr)	Length of expr in UTF-16 code units.
CHAR_LENGTH(expr)	Synonym for LENGTH.
CHARACTER_LENGTH(expr)	Synonym for LENGTH.
STRLEN(expr)	Synonym for LENGTH.
LOOKUP(expr, lookupName)	Look up expr in a registered query-time lookup table.
LOWER(expr)	Returns expr in all lowercase.
REGEXP_EXTRACT(expr, pattern, [index])	Apply regular expression pattern and extract a capture group, or null if there is no match. If index is unspecified or zero, returns the substring that matched the pattern.
REPLACE(expr, pattern, replacement)	Replaces pattern with replacement in expr, and returns the result.
STRPOS(haystack, needle)	Returns the index of needle within haystack, starting from 1. If the needle is not found, returns 0.
SUBSTRING(expr, index, [length])	Returns a substring of expr starting at index, with a max length, both measured in UTF-16 code units.
SUBSTR(expr, index, [length])	Synonym for SUBSTRING.
TRIM([BOTH | LEADING | TRAILING] [<chars> FROM] expr)	Returns expr with characters removed from the leading, trailing, or both ends of "expr" if they are in "chars". If "chars" is not provided, it defaults to " " (a space). If the directional argument is not provided, it defaults to "BOTH".
BTRIM(expr[, chars])	Alternate form of TRIM(BOTH <chars> FROM <expr>).
LTRIM(expr[, chars])	Alternate form of TRIM(LEADING <chars> FROM <expr>).
RTRIM(expr[, chars])	Alternate form of TRIM(TRAILING <chars> FROM <expr>).
UPPER(expr)	Returns expr in all uppercase.

Time functions

Time functions can be used with Druid's __time column, with any column storing millisecond timestamps through use of the MILLIS_TO_TIMESTAMP function, or with any column storing string timestamps through use of the TIME_PARSE function. By default, time operations use the UTC time zone. You can change the time zone by setting the connection context parameter "sqlTimeZone" to the name of another time zone, like "America/Los_Angeles", or to an offset like "-08:00". If you need to mix multiple time zones in the same query, or if you need to use a time zone other than the connection time zone, some functions also accept time zones as parameters. These parameters always take precedence over the connection time zone.

Function	Notes
CURRENT_TIMESTAMP	Current timestamp in the connection's time zone.
CURRENT_DATE	Current date in the connection's time zone.
DATE_TRUNC(<unit>, <timestamp_expr>)	Rounds down a timestamp, returning it as a new timestamp. Unit can be 'milliseconds', 'second', 'minute', 'hour', 'day', 'week', 'month', 'quarter', 'year', 'decade', 'century', or 'millenium'.
TIME_FLOOR(<timestamp_expr>, <period>, [<origin>, [<timezone>]])	Rounds down a timestamp, returning it as a new timestamp. Period can be any ISO8601 period, like P3M (quarters) or PT12H (half-days). The time zone, if provided, should be a time zone name like "America/Los_Angeles" or offset like "-08:00". This function is similar to FLOOR but is more flexible.
TIME_SHIFT(<timestamp_expr>, <period>, <step>, [<timezone>])	Shifts a timestamp by a period (step times), returning it as a new timestamp. Period can be any ISO8601 period. Step may be negative. The time zone, if provided, should be a time zone name like "America/Los_Angeles" or offset like "-08:00".
TIME_EXTRACT(<timestamp_expr>, [<unit>, [<timezone>]])	Extracts a time part from expr, returning it as a number. Unit can be EPOCH, SECOND, MINUTE, HOUR, DAY (day of month), DOW (day of week), DOY (day of year), WEEK (week of week year), MONTH (1 through 12), QUARTER (1 through 4), or YEAR. The time zone, if provided, should be a time zone name like "America/Los_Angeles" or offset like "-08:00". This function is similar to EXTRACT but is more flexible. Unit and time zone must be literals, and must be provided quoted, like TIME_EXTRACT(__time, 'HOUR') or TIME_EXTRACT(__time, 'HOUR', 'America/Los_Angeles').
TIME_PARSE(<string_expr>, [<pattern>, [<timezone>]])	Parses a string into a timestamp using a given Joda DateTimeFormat pattern, or ISO8601 (e.g. 2000-01-02T03:04:05Z) if the pattern is not provided. The time zone, if provided, should be a time zone name like "America/Los_Angeles" or offset like "-08:00", and will be used as the time zone for strings that do not include a time zone offset. Pattern and time zone must be literals. Strings that cannot be parsed as timestamps will be returned as NULL.
TIME_FORMAT(<timestamp_expr>, [<pattern>, [<timezone>]])	Formats a timestamp as a string with a given Joda DateTimeFormat pattern, or ISO8601 (e.g. 2000-01-02T03:04:05Z) if the pattern is not provided. The time zone, if provided, should be a time zone name like "America/Los_Angeles" or offset like "-08:00". Pattern and time zone must be literals.
MILLIS_TO_TIMESTAMP(millis_expr)	Converts a number of milliseconds since the epoch into a timestamp.
TIMESTAMP_TO_MILLIS(timestamp_expr)	Converts a timestamp into a number of milliseconds since the epoch.
EXTRACT(<unit> FROM timestamp_expr)	Extracts a time part from expr, returning it as a number. Unit can be EPOCH, SECOND, MINUTE, HOUR, DAY (day of month), DOW (day of week), DOY (day of year), WEEK (week of year), MONTH, QUARTER, or YEAR. Units must be provided unquoted, like EXTRACT(HOUR FROM __time).
FLOOR(timestamp_expr TO <unit>)	Rounds down a timestamp, returning it as a new timestamp. Unit can be SECOND, MINUTE, HOUR, DAY, WEEK, MONTH, QUARTER, or YEAR.
CEIL(timestamp_expr TO <unit>)	Rounds up a timestamp, returning it as a new timestamp. Unit can be SECOND, MINUTE, HOUR, DAY, WEEK, MONTH, QUARTER, or YEAR.
TIMESTAMPADD(<unit>, <count>, <timestamp>)	Equivalent to timestamp + count * INTERVAL '1' UNIT.
timestamp_expr { + | - } <interval_expr>	Add or subtract an amount of time from a timestamp. interval_expr can include interval literals like INTERVAL '2' HOUR, and may include interval arithmetic as well. This operator treats days as uniformly 86400 seconds long, and does not take into account daylight savings time. To account for daylight savings time, use TIME_SHIFT instead.

Comparison operators

Function	Notes
x = y	Equals.
x <> y	Not-equals.
x > y	Greater than.
x >= y	Greater than or equal to.
x < y	Less than.
x <= y	Less than or equal to.
x BETWEEN y AND z	Equivalent to x >= y AND x <= z.
x NOT BETWEEN y AND z	Equivalent to x < y OR x > z.
x LIKE pattern [ESCAPE esc]	True if x matches a SQL LIKE pattern (with an optional escape).
x NOT LIKE pattern [ESCAPE esc]	True if x does not match a SQL LIKE pattern (with an optional escape).
x IS NULL	True if x is NULL or empty string.
x IS NOT NULL	True if x is neither NULL nor empty string.
x IS TRUE	True if x is true.
x IS NOT TRUE	True if x is not true.
x IS FALSE	True if x is false.
x IS NOT FALSE	True if x is not false.
x IN (values)	True if x is one of the listed values.
x NOT IN (values)	True if x is not one of the listed values.
x IN (subquery)	True if x is returned by the subquery. See Syntax and execution above for details about how Druid SQL handles IN (subquery).
x NOT IN (subquery)	True if x is not returned by the subquery. See Syntax and execution for details about how Druid SQL handles IN (subquery).
x AND y	Boolean AND.
x OR y	Boolean OR.
NOT x	Boolean NOT.

Other functions

Function	Notes
CAST(value AS TYPE)	Cast value to another type. See Data types and casts for details about how Druid SQL handles CAST.
CASE expr WHEN value1 THEN result1 \[ WHEN value2 THEN result2 ... \] \[ ELSE resultN \] END	Simple CASE.
CASE WHEN boolean_expr1 THEN result1 \[ WHEN boolean_expr2 THEN result2 ... \] \[ ELSE resultN \] END	Searched CASE.
NULLIF(value1, value2)	Returns NULL if value1 and value2 match, else returns value1.
COALESCE(value1, value2, ...)	Returns the first value that is neither NULL nor empty string.

Unsupported features

Druid does not support all SQL features, including:

• OVER clauses, and analytic functions such as LAG and LEAD.
• JOIN clauses, other than semi-joins as described above.
• OFFSET clauses.
• DDL and DML.

Additionally, some Druid features are not supported by the SQL language. Some unsupported Druid features include:

• Multi-value dimensions.
• DataSketches aggregators.
• Spatial filters.
• Query cancellation.

Data types and casts

Druid natively supports five basic column types: "long" (64 bit signed int), "float" (32 bit float), "double" (64 bit float) "string" (UTF-8 encoded strings), and "complex" (catch-all for more exotic data types like hyperUnique and approxHistogram columns). Timestamps (including the __time column) are stored as longs, with the value being the number of milliseconds since 1 January 1970 UTC.

At runtime, Druid may widen 32-bit floats to 64-bit for certain operators, like SUM aggregators. The reverse will not happen: 64-bit floats are not be narrowed to 32-bit.

Druid generally treats NULLs and empty strings interchangeably, rather than according to the SQL standard. As such, Druid SQL only has partial support for NULLs. For example, the expressions col IS NULL and col = '' are equivalent, and both will evaluate to true if col contains an empty string. Similarly, the expression COALESCE(col1, col2) will return col2 if col1 is an empty string. While the COUNT(*) aggregator counts all rows, the COUNT(expr) aggregator will count the number of rows where expr is neither null nor the empty string. String columns in Druid are NULLable. Numeric columns are NOT NULL; if you query a numeric column that is not present in all segments of your Druid datasource, then it will be treated as zero for rows from those segments.

For mathematical operations, Druid SQL will use integer math if all operands involved in an expression are integers. Otherwise, Druid will switch to floating point math. You can force this to happen by casting one of your operands to FLOAT.

The following table describes how SQL types map onto Druid types during query runtime. Casts between two SQL types that have the same Druid runtime type will have no effect, other than exceptions noted in the table. Casts between two SQL types that have different Druid runtime types will generate a runtime cast in Druid. If a value cannot be properly cast to another value, as in CAST('foo' AS BIGINT), the runtime will substitute a default value. NULL values cast to non-nullable types will also be substitued with a default value (for example, nulls cast to numbers will be converted to zeroes).

SQL type	Druid runtime type	Default value	Notes
CHAR	STRING	''
VARCHAR	STRING	''	Druid STRING columns are reported as VARCHAR
DECIMAL	DOUBLE	0.0	DECIMAL uses floating point, not fixed point math
FLOAT	FLOAT	0.0	Druid FLOAT columns are reported as FLOAT
REAL	DOUBLE	0.0
DOUBLE	DOUBLE	0.0	Druid DOUBLE columns are reported as DOUBLE
BOOLEAN	LONG	false
TINYINT	LONG	0
SMALLINT	LONG	0
INTEGER	LONG	0
BIGINT	LONG	0	Druid LONG columns (except __time) are reported as BIGINT
TIMESTAMP	LONG	0, meaning 1970-01-01 00:00:00 UTC	Druid's __time column is reported as TIMESTAMP. Casts between string and timestamp types assume standard SQL formatting, e.g. 2000-01-02 03:04:05, not ISO8601 formatting. For handling other formats, use one of the time functions
DATE	LONG	0, meaning 1970-01-01	Casting TIMESTAMP to DATE rounds down the timestamp to the nearest day. Casts between string and date types assume standard SQL formatting, e.g. 2000-01-02. For handling other formats, use one of the time functions
OTHER	COMPLEX	none	May represent various Druid column types such as hyperUnique, approxHistogram, etc

Query execution

Queries without aggregations will use Druid's Scan or Select native query types. Scan is used whenever possible, as it is generally higher performance and more efficient than Select. However, Select is used in one case: when the query includes an ORDER BY __time, since Scan does not have a sorting feature.

Aggregation queries (using GROUP BY, DISTINCT, or any aggregation functions) will use one of Druid's three native aggregation query types. Two (Timeseries and TopN) are specialized for specific types of aggregations, whereas the other (GroupBy) is general-purpose.

• Timeseries is used for queries that GROUP BY FLOOR(__time TO <unit>) or TIME_FLOOR(__time, period), have no other grouping expressions, no HAVING or LIMIT clauses, no nesting, and either no ORDER BY, or an ORDER BY that orders by same expression as present in GROUP BY. It also uses Timeseries for "grand total" queries that have aggregation functions but no GROUP BY. This query type takes advantage of the fact that Druid segments are sorted by time.

• TopN is used by default for queries that group by a single expression, do have ORDER BY and LIMIT clauses, do not have HAVING clauses, and are not nested. However, the TopN query type will deliver approximate ranking and results in some cases; if you want to avoid this, set "useApproximateTopN" to "false". TopN results are always computed in memory. See the TopN documentation for more details.

• GroupBy is used for all other aggregations, including any nested aggregation queries. Druid's GroupBy is a traditional aggregation engine: it delivers exact results and rankings and supports a wide variety of features. GroupBy aggregates in memory if it can, but it may spill to disk if it doesn't have enough memory to complete your query. Results are streamed back from data nodes through the broker if you ORDER BY the same expressions in your GROUP BY clause, or if you don't have an ORDER BY at all. If your query has an ORDER BY referencing expressions that don't appear in the GROUP BY clause (like aggregation functions) then the broker will materialize a list of results in memory, up to a max of your LIMIT, if any. See the GroupBy documentation for details about tuning performance and memory use.

If your query does nested aggregations (an aggregation subquery in your FROM clause) then Druid will execute it as a nested GroupBy. In nested GroupBys, the innermost aggregation is distributed, but all outer aggregations beyond that take place locally on the query broker.

Semi-join queries containing WHERE clauses like col IN (SELECT expr FROM ...) are executed with a special process. The broker will first translate the subquery into a GroupBy to find distinct values of expr. Then, the broker will rewrite the subquery to a literal filter, like col IN (val1, val2, ...) and run the outer query. The configuration parameter druid.sql.planner.maxSemiJoinRowsInMemory controls the maximum number of values that will be materialized for this kind of plan.

For all native query types, filters on the __time column will be translated into top-level query "intervals" whenever possible, which allows Druid to use its global time index to quickly prune the set of data that must be scanned. In addition, Druid will use indexes local to each data node to further speed up WHERE evaluation. This can typically be done for filters that involve boolean combinations of references to and functions of single columns, like WHERE col1 = 'a' AND col2 = 'b', but not WHERE col1 = col2.

Approximate algorithms

Druid SQL will use approximate algorithms in some situations:

• The COUNT(DISTINCT col) aggregation functions by default uses a variant of HyperLogLog, a fast approximate distinct counting algorithm. Druid SQL will switch to exact distinct counts if you set "useApproximateCountDistinct" to "false", either through query context or through broker configuration.
• GROUP BY queries over a single column with ORDER BY and LIMIT may be executed using the TopN engine, which uses an approximate algorithm. Druid SQL will switch to an exact grouping algorithm if you set "useApproximateTopN" to "false", either through query context or through broker configuration.
• The APPROX_COUNT_DISTINCT and APPROX_QUANTILE aggregation functions always use approximate algorithms, regardless of configuration.

Client APIs

JSON over HTTP

You can make Druid SQL queries using JSON over HTTP by posting to the endpoint /druid/v2/sql/. The request should be a JSON object with a "query" field, like {"query" : "SELECT COUNT(*) FROM data_source WHERE foo = 'bar'"}.

Results are available in two formats: "object" (the default; a JSON array of JSON objects), and "array" (a JSON array of JSON arrays). In "object" form, each row's field names will match the column names from your SQL query. In "array" form, each row's values are returned in the order specified in your SQL query.

You can use curl to send SQL queries from the command-line:

$ cat query.json
{"query":"SELECT COUNT(*) AS TheCount FROM data_source"}

$ curl -XPOST -H'Content-Type: application/json' http://BROKER:8082/druid/v2/sql/ -d @query.json
[{"TheCount":24433}]

Metadata is available over the HTTP API by querying the "INFORMATION_SCHEMA" tables.

Finally, you can also provide connection context parameters by adding a "context" map, like:

{
  "query" : "SELECT COUNT(*) FROM data_source WHERE foo = 'bar' AND __time > TIMESTAMP '2000-01-01 00:00:00'",
  "context" : {
    "sqlTimeZone" : "America/Los_Angeles"
  }
}

JDBC

You can make Druid SQL queries using the Avatica JDBC driver. Once you've downloaded the Avatica client jar, add it to your classpath and use the connect string jdbc:avatica:remote:url=http://BROKER:8082/druid/v2/sql/avatica/.

Example code:

// Connect to /druid/v2/sql/avatica/ on your broker.
String url = "jdbc:avatica:remote:url=http://localhost:8082/druid/v2/sql/avatica/";

// Set any connection context parameters you need here (see "Connection context" below).
// Or leave empty for default behavior.
Properties connectionProperties = new Properties();

try (Connection connection = DriverManager.getConnection(url, connectionProperties)) {
  try (
      final Statement statement = client.createStatement();
      final ResultSet resultSet = statement.executeQuery(query)
  ) {
    while (resultSet.next()) {
      // Do something
    }
  }
}

Table metadata is available over JDBC using connection.getMetaData() or by querying the "INFORMATION_SCHEMA" tables. Parameterized queries (using ? or other placeholders) don't work properly, so avoid those.

Connection Stickiness

Druid's JDBC server does not share connection state between brokers. This means that if you're using JDBC and have multiple Druid brokers, you should either connect to a specific broker, or use a load balancer with sticky sessions enabled.

The Druid Router node provides connection stickiness when balancing JDBC requests. Please see Router documentation for more details.

Note that the non-JDBC JSON over HTTP API is stateless and does not require stickiness.

Connection context

Druid SQL supports setting connection parameters on the client. The parameters in the table below affect SQL planning. All other context parameters you provide will be attached to Druid queries and can affect how they run. See Query context for details on the possible options.

Connection context can be specified as JDBC connection properties or as a "context" object in the JSON API.

Parameter	Description	Default value
sqlTimeZone	Sets the time zone for this connection, which will affect how time functions and timestamp literals behave. Should be a time zone name like "America/Los_Angeles" or offset like "-08:00".	UTC
useApproximateCountDistinct	Whether to use an approximate cardinalty algorithm for COUNT(DISTINCT foo).	druid.sql.planner.useApproximateCountDistinct on the broker
useApproximateTopN	Whether to use approximate TopN queries when a SQL query could be expressed as such. If false, exact GroupBy queries will be used instead.	druid.sql.planner.useApproximateTopN on the broker
useFallback	Whether to evaluate operations on the broker when they cannot be expressed as Druid queries. This option is not recommended for production since it can generate unscalable query plans. If false, SQL queries that cannot be translated to Druid queries will fail.	druid.sql.planner.useFallback on the broker

Retrieving metadata

Druid brokers infer table and column metadata for each dataSource from segments loaded in the cluster, and use this to plan SQL queries. This metadata is cached on broker startup and also updated periodically in the background through SegmentMetadata queries. Background metadata refreshing is triggered by segments entering and exiting the cluster, and can also be throttled through configuration.

You can access table and column metadata through JDBC using connection.getMetaData(), or through the INFORMATION_SCHEMA tables described below. For example, to retrieve metadata for the Druid datasource "foo", use the query:

SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_SCHEMA = 'druid' AND TABLE_NAME = 'foo'

SCHEMATA table

Column	Notes
CATALOG_NAME	Unused
SCHEMA_NAME
SCHEMA_OWNER	Unused
DEFAULT_CHARACTER_SET_CATALOG	Unused
DEFAULT_CHARACTER_SET_SCHEMA	Unused
DEFAULT_CHARACTER_SET_NAME	Unused
SQL_PATH	Unused

TABLES table

Column	Notes
TABLE_CATALOG	Unused
TABLE_SCHEMA
TABLE_NAME
TABLE_TYPE	"TABLE" or "SYSTEM_TABLE"

COLUMNS table

Column	Notes
TABLE_CATALOG	Unused
TABLE_SCHEMA
TABLE_NAME
COLUMN_NAME
ORDINAL_POSITION
COLUMN_DEFAULT	Unused
IS_NULLABLE
DATA_TYPE
CHARACTER_MAXIMUM_LENGTH	Unused
CHARACTER_OCTET_LENGTH	Unused
NUMERIC_PRECISION
NUMERIC_PRECISION_RADIX
NUMERIC_SCALE
DATETIME_PRECISION
CHARACTER_SET_NAME
COLLATION_NAME
JDBC_TYPE	Type code from java.sql.Types (Druid extension)

Server configuration

The Druid SQL server is configured through the following properties on the broker.

Property	Description	Default
druid.sql.enable	Whether to enable SQL at all, including background metadata fetching. If false, this overrides all other SQL-related properties and disables SQL metadata, serving, and planning completely.	false
druid.sql.avatica.enable	Whether to enable JDBC querying at /druid/v2/sql/avatica/.	true
druid.sql.avatica.maxConnections	Maximum number of open connections for the Avatica server. These are not HTTP connections, but are logical client connections that may span multiple HTTP connections.	50
druid.sql.avatica.maxRowsPerFrame	Maximum number of rows to return in a single JDBC frame. Setting this property to -1 indicates that no row limit should be applied. Clients can optionally specify a row limit in their requests; if a client specifies a row limit, the lesser value of the client-provided limit and maxRowsPerFrame will be used.	100,000
druid.sql.avatica.maxStatementsPerConnection	Maximum number of simultaneous open statements per Avatica client connection.	1
druid.sql.avatica.connectionIdleTimeout	Avatica client connection idle timeout.	PT5M
druid.sql.http.enable	Whether to enable JSON over HTTP querying at /druid/v2/sql/.	true
druid.sql.planner.maxQueryCount	Maximum number of queries to issue, including nested queries. Set to 1 to disable sub-queries, or set to 0 for unlimited.	8
druid.sql.planner.maxSemiJoinRowsInMemory	Maximum number of rows to keep in memory for executing two-stage semi-join queries like SELECT * FROM Employee WHERE DeptName IN (SELECT DeptName FROM Dept).	100000
druid.sql.planner.maxTopNLimit	Maximum threshold for a TopN query. Higher limits will be planned as GroupBy queries instead.	100000
druid.sql.planner.metadataRefreshPeriod	Throttle for metadata refreshes.	PT1M
druid.sql.planner.selectPageSize	Page size threshold for Select queries. Select queries for larger resultsets will be issued back-to-back using pagination.	1000
druid.sql.planner.useApproximateCountDistinct	Whether to use an approximate cardinalty algorithm for COUNT(DISTINCT foo).	true
druid.sql.planner.useApproximateTopN	Whether to use approximate TopN queries when a SQL query could be expressed as such. If false, exact GroupBy queries will be used instead.	true
druid.sql.planner.useFallback	Whether to evaluate operations on the broker when they cannot be expressed as Druid queries. This option is not recommended for production since it can generate unscalable query plans. If false, SQL queries that cannot be translated to Druid queries will fail.	false