SQL-based ingestion reference

This page describes SQL-based batch ingestion using the druid-multi-stage-query extension, new in Druid 24.0. Refer to the ingestion methods table to determine which ingestion method is right for you.

SQL reference

This topic is a reference guide for the multi-stage query architecture in Apache Druid. For examples of real-world usage, refer to the Examples page.

INSERT and REPLACE load data into a Druid datasource from either an external input source, or from another datasource. When loading from an external datasource, you typically must provide the kind of input source, the data format, and the schema (signature) of the input file. Druid provides table functions to allow you to specify the external file. There are two kinds. EXTERN works with the JSON-serialized specs for the three items, using the same JSON you would use in native ingest. A set of other, input-source-specific functions use SQL syntax to specify the format and the input schema. There is one function for each input source. The input-source-specific functions allow you to use SQL query parameters to specify the set of files (or URIs), making it easy to reuse the same SQL statement for each ingest: just specify the set of files to use each time.

`EXTERN` Function

Use the EXTERN function to read external data. The function has two variations.

Function variation 1, with the input schema expressed as JSON:

SELECT
 <column>
FROM TABLE(
  EXTERN(
    '<Druid input source>',
    '<Druid input format>',
    '<row signature>'
  )
)

EXTERN consists of the following parts:

Any Druid input source as a JSON-encoded string.
Any Druid input format as a JSON-encoded string.
A row signature, as a JSON-encoded array of column descriptors. Each column descriptor must have a name and a type. The type can be string, long, double, or float. This row signature is used to map the external data into the SQL layer.

Variation 2, with the input schema expressed in SQL using an EXTEND clause. (See the next section for more detail on EXTEND). This format also uses named arguments to make the SQL a bit easier to read:

SELECT
 <column>
FROM TABLE(
  EXTERN(
    inputSource => '<Druid input source>',
    inputFormat => '<Druid input format>'
  )) (<columns>)

The input source and format are as above. The columns are expressed as in a SQL CREATE TABLE. Example: (timestamp VARCHAR, metricType VARCHAR, value BIGINT). The optional EXTEND keyword can precede the column list: EXTEND (timestamp VARCHAR...).

For more information, see Read external data with EXTERN.

`INSERT`

Use the INSERT statement to insert data.

Unlike standard SQL, INSERT loads data into the target table according to column name, not positionally. If necessary, use AS in your SELECT column list to assign the correct names. Do not rely on their positions within the SELECT clause.

Statement format:

INSERT INTO <table name>
< SELECT query >
PARTITIONED BY <time frame>
[ CLUSTERED BY <column list> ]

INSERT consists of the following parts:

Optional context parameters.
An INSERT INTO <dataSource> clause at the start of your query, such as INSERT INTO your-table.
A clause for the data you want to insert, such as SELECT ... FROM .... You can use EXTERN to reference external tables using FROM TABLE(EXTERN(...)).
A PARTITIONED BY clause, such as PARTITIONED BY DAY.
An optional CLUSTERED BY clause.

For more information, see Load data with INSERT.

`REPLACE`

You can use the REPLACE function to replace all or some of the data.

Unlike standard SQL, REPLACE loads data into the target table according to column name, not positionally. If necessary, use AS in your SELECT column list to assign the correct names. Do not rely on their positions within the SELECT clause.

`REPLACE` all data

Function format to replace all data:

REPLACE INTO <target table>
OVERWRITE ALL
< SELECT query >
PARTITIONED BY <time granularity>
[ CLUSTERED BY <column list> ]

`REPLACE` specific time ranges

Function format to replace specific time ranges:

REPLACE INTO <target table>
OVERWRITE WHERE __time >= TIMESTAMP '<lower bound>' AND __time < TIMESTAMP '<upper bound>'
< SELECT query >
PARTITIONED BY <time granularity>
[ CLUSTERED BY <column list> ]

REPLACE consists of the following parts:

Optional context parameters.
A REPLACE INTO <dataSource> clause at the start of your query, such as REPLACE INTO "your-table".
An OVERWRITE clause after the datasource, either OVERWRITE ALL or OVERWRITE WHERE:
- OVERWRITE ALL replaces the entire existing datasource with the results of the query.
- OVERWRITE WHERE drops the time segments that match the condition you set. Conditions are based on the __time column and use the format __time [< > = <= >=] TIMESTAMP. Use them with AND, OR, and NOT between them, inclusive of the timestamps specified. No other expressions or functions are valid in OVERWRITE.
A clause for the actual data you want to use for the replacement.
A PARTITIONED BY clause, such as PARTITIONED BY DAY.
An optional CLUSTERED BY clause.

For more information, see Overwrite data with REPLACE.

`PARTITIONED BY`

The PARTITIONED BY <time granularity> clause is required for INSERT and REPLACE. See Partitioning for details.

The following granularity arguments are accepted:

Time unit keywords: HOUR, DAY, MONTH, or YEAR. Equivalent to FLOOR(__time TO TimeUnit).
Time units as ISO 8601 period strings: :'PT1H', 'P1D, etc. (Druid 26.0 and later.)
TIME_FLOOR(__time, 'granularity_string'), where granularity_string is one of the ISO 8601 periods listed below. The first argument must be __time.
FLOOR(__time TO TimeUnit), where TimeUnit is any unit supported by the FLOOR function. The first argument must be __time.
ALL or ALL TIME, which effectively disables time partitioning by placing all data in a single time chunk. To use LIMIT or OFFSET at the outer level of your INSERT or REPLACE query, you must set PARTITIONED BY to ALL or ALL TIME.

Earlier versions required the TIME_FLOOR notation to specify a granularity other than the keywords. In the current version, the string constant provides a simpler equivalent solution.

The following ISO 8601 periods are supported for TIME_FLOOR and the string constant:

PT1S
PT1M
PT5M
PT10M
PT15M
PT30M
PT1H
PT6H
P1D
P1W
P1M
P3M
P1Y

For more information about partitioning, see Partitioning.

`CLUSTERED BY`

The CLUSTERED BY <column list> clause is optional for INSERT and REPLACE. It accepts a list of column names or expressions.

For more information about clustering, see Clustering.

Context parameters

In addition to the Druid SQL context parameters, the multi-stage query task engine accepts certain context parameters that are specific to it.

Use context parameters alongside your queries to customize the behavior of the query. If you're using the API, include the context parameters in the query context when you submit a query:

{
  "query": "SELECT 1 + 1",
  "context": {
    "<key>": "<value>",
    "maxNumTasks": 3
  }
}

If you're using the web console, you can specify the context parameters through various UI options.

The following table lists the context parameters for the MSQ task engine:

Parameter	Description	Default value
`maxNumTasks`	SELECT, INSERT, REPLACE The maximum total number of tasks to launch, including the controller task. The lowest possible value for this setting is 2: one controller and one worker. All tasks must be able to launch simultaneously. If they cannot, the query returns a `TaskStartTimeout` error code after approximately 10 minutes. May also be provided as `numTasks`. If both are present, `maxNumTasks` takes priority.	2
`taskAssignment`	SELECT, INSERT, REPLACE Determines how many tasks to use. Possible values include: `max`: Uses as many tasks as possible, up to `maxNumTasks`. `auto`: When file sizes can be determined through directory listing (for example: local files, S3, GCS, HDFS) uses as few tasks as possible without exceeding 10 GiB or 10,000 files per task, unless exceeding these limits is necessary to stay within `maxNumTasks`. When file sizes cannot be determined through directory listing (for example: http), behaves the same as `max`.	`max`
`finalizeAggregations`	SELECT, INSERT, REPLACE Determines the type of aggregation to return. If true, Druid finalizes the results of complex aggregations that directly appear in query results. If false, Druid returns the aggregation's intermediate type rather than finalized type. This parameter is useful during ingestion, where it enables storing sketches directly in Druid tables. For more information about aggregations, see SQL aggregation functions.	true
`sqlJoinAlgorithm`	SELECT, INSERT, REPLACE Algorithm to use for JOIN. Use `broadcast` (the default) for broadcast hash join or `sortMerge` for sort-merge join. Affects all JOIN operations in the query. See Joins for more details.	`broadcast`
`rowsInMemory`	INSERT or REPLACE Maximum number of rows to store in memory at once before flushing to disk during the segment generation process. Ignored for non-INSERT queries. In most cases, use the default value. You may need to override the default if you run into one of the known issues around memory usage.	100,000
`segmentSortOrder`	INSERT or REPLACE Normally, Druid sorts rows in individual segments using `__time` first, followed by the CLUSTERED BY clause. When you set `segmentSortOrder`, Druid sorts rows in segments using this column list first, followed by the CLUSTERED BY order. You provide the column list as comma-separated values or as a JSON array in string form. If your query includes `__time`, then this list must begin with `__time`. For example, consider an INSERT query that uses `CLUSTERED BY country` and has `segmentSortOrder` set to `__time,city`. Within each time chunk, Druid assigns rows to segments based on `country`, and then within each of those segments, Druid sorts those rows by `__time` first, then `city`, then `country`.	empty list
`maxParseExceptions`	SELECT, INSERT, REPLACE Maximum number of parse exceptions that are ignored while executing the query before it stops with `TooManyWarningsFault`. To ignore all the parse exceptions, set the value to -1.	0
`rowsPerSegment`	INSERT or REPLACE The number of rows per segment to target. The actual number of rows per segment may be somewhat higher or lower than this number. In most cases, use the default. For general information about sizing rows per segment, see Segment Size Optimization.	3,000,000
`indexSpec`	INSERT or REPLACE An `indexSpec` to use when generating segments. May be a JSON string or object. See Front coding for details on configuring an `indexSpec` with front coding.	See `indexSpec`.
`durableShuffleStorage`	SELECT, INSERT, REPLACE Whether to use durable storage for shuffle mesh. To use this feature, configure the durable storage at the server level using `druid.msq.intermediate.storage.enable=true`). If these properties are not configured, any query with the context variable `durableShuffleStorage=true` fails with a configuration error.	`false`
`faultTolerance`	SELECT, INSERT, REPLACE Whether to turn on fault tolerance mode or not. Failed workers are retried based on Limits. Cannot be used when `durableShuffleStorage` is explicitly set to false.	`false`

Joins

Joins in multi-stage queries use one of two algorithms based on what you set the context parameter sqlJoinAlgorithm to:

broadcast (default)
sortMerge.

If you omit this context parameter, the MSQ task engine uses broadcast since it's the default join algorithm. The context parameter applies to the entire SQL statement, so you can't mix different join algorithms in the same query.

Broadcast

The default join algorithm for multi-stage queries is a broadcast hash join, which is similar to how joins are executed with native queries.

To use broadcast joins, either omit the sqlJoinAlgorithm or set it to broadcast.

For a broadcast join, any adjacent joins are flattened into a structure with a "base" input (the bottom-leftmost one) and other leaf inputs (the rest). Next, any subqueries that are inputs the join (either base or other leafs) are planned into independent stages. Then, the non-base leaf inputs are all connected as broadcast inputs to the "base" stage.

Together, all of these non-base leaf inputs must not exceed the limit on broadcast table footprint. There is no limit on the size of the base (leftmost) input.

Only LEFT JOIN, INNER JOIN, and CROSS JOIN are supported with with broadcast.

Join conditions, if present, must be equalities. It is not necessary to include a join condition; for example, CROSS JOIN and comma join do not require join conditions.

The following example has a single join chain where orders is the base input while products and customers are non-base leaf inputs. The broadcast inputs (products and customers) must fall under the limit on broadcast table footprint, but the base orders input can be unlimited in size.

The query reads products and customers and then broadcasts both to the stage that reads orders. That stage loads the broadcast inputs (products and customers) in memory and walks through orders row by row. The results are aggregated and written to the table orders_enriched.

REPLACE INTO orders_enriched
OVERWRITE ALL
SELECT
  orders.__time,
  products.name AS product_name,
  customers.name AS customer_name,
  SUM(orders.amount) AS amount
FROM orders
LEFT JOIN products ON orders.product_id = products.id
LEFT JOIN customers ON orders.customer_id = customers.id
GROUP BY 1, 2
PARTITIONED BY HOUR
CLUSTERED BY product_name

Sort-merge

You can use the sort-merge join algorithm to make queries more scalable at the cost of performance. If your goal is performance, consider broadcast joins. There are various scenarios where broadcast join would return a BroadcastTablesTooLarge error, but a sort-merge join would succeed.

To use the sort-merge join algorithm, set the context parameter sqlJoinAlgorithm to sortMerge.

In a sort-merge join, each pairwise join is planned into its own stage with two inputs. The two inputs are partitioned and sorted using a hash partitioning on the same key.

When using the sort-merge algorithm, keep the following in mind:

There is no limit on the overall size of either input, so sort-merge is a good choice for performing a join of two large inputs or for performing a self-join of a large input with itself.
There is a limit on the amount of data associated with each individual key. If both sides of the join exceed this limit, the query returns a TooManyRowsWithSameKey error. If only one side exceeds the limit, the query does not return this error.
Join conditions are optional but must be equalities if they are present. For example, CROSS JOIN and comma join do not require join conditions.
All join types are supported with sortMerge: LEFT, RIGHT, INNER, FULL, and CROSS.

The following example runs using a single sort-merge join stage that receives eventstream (partitioned on user_id) and users (partitioned on id) as inputs. There is no limit on the size of either input.

REPLACE INTO eventstream_enriched
OVERWRITE ALL
SELECT
  eventstream.__time,
  eventstream.user_id,
  eventstream.event_type,
  eventstream.event_details,
  users.signup_date AS user_signup_date
FROM eventstream
LEFT JOIN users ON eventstream.user_id = users.id
PARTITIONED BY HOUR
CLUSTERED BY user

The context parameter that sets sqlJoinAlgorithm to sortMerge is not shown in the above example.

Durable Storage

Using durable storage with your SQL-based ingestions can improve their reliability by writing intermediate files to a storage location temporarily.

To prevent durable storage from getting filled up with temporary files in case the tasks fail to clean them up, a periodic cleaner can be scheduled to clean the directories corresponding to which there isn't a controller task running. It utilizes the storage connector to work upon the durable storage. The durable storage location should only be utilized to store the output for cluster's MSQ tasks. If the location contains other files or directories, then they will get cleaned up as well.

Enabling durable storage also enables the use of local disk to store temporary files, such as the intermediate files produced by the super sorter. The limit set by druid.indexer.task.tmpStorageBytesPerTask for maximum number of bytes of local storage to be used per task will be respected by MSQ tasks. If the configured limit is too low, NotEnoughTemporaryStorageFault may be thrown.

Enable durable storage

Imply Manager automatically configures durable storage for you if you use S3 as deep storage. No configuration is required on your part.

To enable durable storage, you need to set the following common service properties:

druid.msq.intermediate.storage.enable=true
druid.msq.intermediate.storage.type=s3
druid.msq.intermediate.storage.bucket=YOUR_BUCKET
druid.msq.intermediate.storage.prefix=YOUR_PREFIX
druid.msq.intermediate.storage.tempDir=/path/to/your/temp/dir

For detailed information about the settings related to durable storage, see Durable storage configurations.

Use durable storage for queries

When you run a query, include the context parameter durableShuffleStorage and set it to true.

For queries where you want to use fault tolerance for workers, set faultTolerance to true, which automatically sets durableShuffleStorage to true.

Durable storage configurations

The following common service properties control how durable storage behaves:

Parameter	Default	Description
`druid.msq.intermediate.storage.bucket`	n/a	The bucket in S3 where you want to store intermediate files.
`druid.msq.intermediate.storage.chunkSize`	100MiB	Optional. Defines the size of each chunk to temporarily store in `druid.msq.intermediate.storage.tempDir`. The chunk size must be between 5 MiB and 5 GiB. A large chunk size reduces the API calls made to the durable storage, however it requires more disk space to store the temporary chunks. Druid uses a default of 100MiB if the value is not provided.
`druid.msq.intermediate.storage.enable`	true	Required. Whether to enable durable storage for the cluster.
`druid.msq.intermediate.storage.maxRetry`	10	Optional. Defines the max number times to attempt S3 API calls to avoid failures due to transient errors.
`druid.msq.intermediate.storage.prefix`	n/a	S3 prefix to store intermediate stage results. Provide a unique value for the prefix. Don't share the same prefix between clusters. If the location includes other files or directories, then they will get cleaned up as well.
`druid.msq.intermediate.storage.tempDir`	n/a	Required. Directory path on the local disk to temporarily store intermediate stage results.
`druid.msq.intermediate.storage.type`	`s3` if your deep storage is S3	Required. The type of storage to use. You can either set this to `local` or `s3`.

In addition to the common service properties, there are certain properties that you configure on the Overlord specifically to clean up intermediate files:

Parameter	Default	Description
`druid.msq.intermediate.storage.cleaner.enabled`	false	Optional. Whether durable storage cleaner should be enabled for the cluster.
`druid.msq.intermediate.storage.cleaner.delaySeconds`	86400	Optional. The delay (in seconds) after the last run post which the durable storage cleaner would clean the outputs.

Limits

Knowing the limits for the MSQ task engine can help you troubleshoot any errors that you encounter. Many of the errors occur as a result of reaching a limit.

The following table lists query limits:

Limit	Value	Error if exceeded
Size of an individual row written to a frame. Row size when written to a frame may differ from the original row size.	1 MB	`RowTooLarge`
Number of segment-granular time chunks encountered during ingestion.	5,000	`TooManyBuckets`
Number of input files/segments per worker.	10,000	`TooManyInputFiles`
Number of output partitions for any one stage. Number of segments generated during ingestion.	25,000	`TooManyPartitions`
Number of output columns for any one stage.	2,000	`TooManyColumns`
Number of cluster by columns that can appear in a stage	1,500	`TooManyClusteredByColumns`
Number of workers for any one stage.	Hard limit is 1,000. Memory-dependent soft limit may be lower.	`TooManyWorkers`
Maximum memory occupied by broadcasted tables.	30% of each processor memory bundle.	`BroadcastTablesTooLarge`
Maximum memory occupied by buffered data during sort-merge join. Only relevant when `sqlJoinAlgorithm` is `sortMerge`.	10 MB	`TooManyRowsWithSameKey`
Maximum relaunch attempts per worker. Initial run is not a relaunch. The worker will be spawned 1 + `workerRelaunchLimit` times before the job fails.	2	`TooManyAttemptsForWorker`
Maximum relaunch attempts for a job across all workers.	100	`TooManyAttemptsForJob`

Error codes

The following table describes error codes you may encounter in the multiStageQuery.payload.status.errorReport.error.errorCode field:

Code	Meaning	Additional fields
`BroadcastTablesTooLarge`	The size of the broadcast tables used in the right hand side of the join exceeded the memory reserved for them in a worker task. Try increasing the peon memory or reducing the size of the broadcast tables.	`maxBroadcastTablesSize`: Memory reserved for the broadcast tables, measured in bytes.
`Canceled`	The query was canceled. Common reasons for cancellation: User-initiated shutdown of the controller task via the `/druid/indexer/v1/task/{taskId}/shutdown` API. Restart or failure of the server process that was running the controller task.
`CannotParseExternalData`	A worker task could not parse data from an external datasource.	`errorMessage`: More details on why parsing failed.
`ColumnNameRestricted`	The query uses a restricted column name.	`columnName`: The restricted column name.
`ColumnTypeNotSupported`	The column type is not supported. This can be because: Support for writing or reading from a particular column type is not supported. The query attempted to use a column type that is not supported by the frame format. This occurs with ARRAY types, which are not yet implemented for frames.	`columnName`: The column name with an unsupported type. `columnType`: The unknown column type.
`InsertCannotAllocateSegment`	The controller task could not allocate a new segment ID due to conflict with existing segments or pending segments. Common reasons for such conflicts: Attempting to mix different granularities in the same intervals of the same datasource. Prior ingestions that used non-extendable shard specs.	`dataSource` `interval`: The interval for the attempted new segment allocation.
`InsertCannotBeEmpty`	An INSERT or REPLACE query did not generate any output rows in a situation where output rows are required for success. This can happen for INSERT or REPLACE queries with `PARTITIONED BY` set to something other than `ALL` or `ALL TIME`.	`dataSource`
`InsertCannotOrderByDescending`	An INSERT query contained a `CLUSTERED BY` expression in descending order. Druid's segment generation code only supports ascending order.	`columnName`
`InsertLockPreempted`	An INSERT or REPLACE query was canceled by a higher-priority ingestion job, such as a real-time ingestion task.
`InsertTimeNull`	An INSERT or REPLACE query encountered a null timestamp in the `__time` field. This can happen due to using an expression like `TIME_PARSE(timestamp) AS __time` with a timestamp that cannot be parsed. (`TIME_PARSE` returns null when it cannot parse a timestamp.) In this case, try parsing your timestamps using a different function or pattern. Or, if your timestamps may genuinely be null, consider using `COALESCE` to provide a default value. One option is `CURRENT_TIMESTAMP`, which represents the start time of the job.
`InsertTimeOutOfBounds`	A REPLACE query generated a timestamp outside the bounds of the TIMESTAMP parameter for your OVERWRITE WHERE clause. To avoid this error, verify that the you specified is valid.	`interval`: time chunk interval corresponding to the out-of-bounds timestamp
`InvalidNullByte`	A string column included a null byte. Null bytes in strings are not permitted.	`column`: The column that included the null byte
`QueryNotSupported`	QueryKit could not translate the provided native query to a multi-stage query. This can happen if the query uses features that aren't supported, like GROUPING SETS.
`QueryRuntimeError`	MSQ uses the native query engine to run the leaf stages. This error tells MSQ that error is in native query runtime. Since this is a generic error, the user needs to look at logs for the error message and stack trace to figure out the next course of action. If the user is stuck, consider raising a `github` issue for assistance.	`baseErrorMessage` error message from the native query runtime.
`RowTooLarge`	The query tried to process a row that was too large to write to a single frame. See the Limits table for specific limits on frame size. Note that the effective maximum row size is smaller than the maximum frame size due to alignment considerations during frame writing.	`maxFrameSize`: The limit on the frame size.
`TaskStartTimeout`	Unable to launch `numTasksNotStarted` worker out of total `totalTasks` workers tasks within `timeout` seconds of the last successful worker launch. There may be insufficient available slots to start all the worker tasks simultaneously. Try splitting up your query into smaller chunks using a smaller value of `maxNumTasks`. Another option is to increase capacity.	`numTasksNotStarted`: Number of tasks not yet started. `totalTasks`: The number of tasks attempted to launch. `timeout`: Timeout, in milliseconds, that was exceeded.
`TooManyAttemptsForJob`	Total relaunch attempt count across all workers exceeded max relaunch attempt limit. See the Limits table for the specific limit.	`maxRelaunchCount`: Max number of relaunches across all the workers defined in the Limits section. `currentRelaunchCount`: current relaunch counter for the job across all workers. `taskId`: Latest task id which failed `rootErrorMessage`: Error message of the latest failed task.
`TooManyAttemptsForWorker`	Worker exceeded maximum relaunch attempt count as defined in the Limits section.	`maxPerWorkerRelaunchCount`: Max number of relaunches allowed per worker as defined in the Limits section. `workerNumber`: the worker number for which the task failed `taskId`: Latest task id which failed `rootErrorMessage`: Error message of the latest failed task.
`TooManyBuckets`	Exceeded the maximum number of partition buckets for a stage (5,000 partition buckets). < br />Partition buckets are created for each `PARTITIONED BY` time chunk for INSERT and REPLACE queries. The most common reason for this error is that your `PARTITIONED BY` is too narrow relative to your data.	`maxBuckets`: The limit on partition buckets.
`TooManyInputFiles`	Exceeded the maximum number of input files or segments per worker (10,000 files or segments). If you encounter this limit, consider adding more workers, or breaking up your query into smaller queries that process fewer files or segments per query.	`numInputFiles`: The total number of input files/segments for the stage. `maxInputFiles`: The maximum number of input files/segments per worker per stage. `minNumWorker`: The minimum number of workers required for a successful run.
`TooManyPartitions`	Exceeded the maximum number of partitions for a stage (25,000 partitions). This can occur with INSERT or REPLACE statements that generate large numbers of segments, since each segment is associated with a partition. If you encounter this limit, consider breaking up your INSERT or REPLACE statement into smaller statements that process less data per statement.	`maxPartitions`: The limit on partitions which was exceeded
`TooManyClusteredByColumns`	Exceeded the maximum number of clustering columns for a stage (1,500 columns). This can occur with `CLUSTERED BY`, `ORDER BY`, or `GROUP BY` with a large number of columns.	`numColumns`: The number of columns requested. `maxColumns`: The limit on columns which was exceeded.`stage`: The stage number exceeding the limit
`TooManyRowsWithSameKey`	The number of rows for a given key exceeded the maximum number of buffered bytes on both sides of a join. See the Limits table for the specific limit. Only occurs when `sqlJoinAlgorithm` is `sortMerge`.	`key`: The key that had a large number of rows. `numBytes`: Number of bytes buffered, which may include other keys. `maxBytes`: Maximum number of bytes buffered.
`TooManyColumns`	Exceeded the maximum number of columns for a stage (2,000 columns).	`numColumns`: The number of columns requested. `maxColumns`: The limit on columns which was exceeded.
`TooManyWarnings`	Exceeded the maximum allowed number of warnings of a particular type.	`rootErrorCode`: The error code corresponding to the exception that exceeded the required limit. `maxWarnings`: Maximum number of warnings that are allowed for the corresponding `rootErrorCode`.
`TooManyWorkers`	Exceeded the maximum number of simultaneously-running workers. See the Limits table for more details.	`workers`: The number of simultaneously running workers that exceeded a hard or soft limit. This may be larger than the number of workers in any one stage if multiple stages are running simultaneously. `maxWorkers`: The hard or soft limit on workers that was exceeded. If this is lower than the hard limit (1,000 workers), then you can increase the limit by adding more memory to each task.
`NotEnoughMemory`	Insufficient memory to launch a stage.	`suggestedServerMemory`: Suggested number of bytes of memory to allocate to a given process. `serverMemory`: The number of bytes of memory available to a single process. `usableMemory`: The number of usable bytes of memory for a single process. `serverWorkers`: The number of workers running in a single process. `serverThreads`: The number of threads in a single process.
`NotEnoughTemporaryStorage`	Insufficient temporary storage configured to launch a stage. This limit is set by the property `druid.indexer.task.tmpStorageBytesPerTask`. This property should be increased to the minimum suggested limit to resolve this.	`suggestedMinimumStorage`: Suggested number of bytes of temporary storage space to allocate to a given process. `configuredTemporaryStorage`: The number of bytes of storage currently configured.
`WorkerFailed`	A worker task failed unexpectedly.	`errorMsg` `workerTaskId`: The ID of the worker task.
`WorkerRpcFailed`	A remote procedure call to a worker task failed and could not recover.	`workerTaskId`: the id of the worker task
`UnknownError`	All other errors.	`message`