# The Influx Query Language Specification ## Introduction This is a reference for the Influx Query Language ("InfluxQL"). InfluxQL is a SQL-like query language for interacting with InfluxDB. It has been lovingly crafted to feel familiar to those coming from other SQL or SQL-like environments while providing features specific to storing and analyzing time series data. ## Notation The syntax is specified using Extended Backus-Naur Form ("EBNF"). EBNF is the same notation used in the [Go](http://golang.org) programming language specification, which can be found [here](https://golang.org/ref/spec). Not so coincidentally, InfluxDB is written in Go. ``` Production = production_name "=" [ Expression ] "." . Expression = Alternative { "|" Alternative } . Alternative = Term { Term } . Term = production_name | token [ "…" token ] | Group | Option | Repetition . Group = "(" Expression ")" . Option = "[" Expression "]" . Repetition = "{" Expression "}" . ``` Notation operators in order of increasing precedence: ``` | alternation () grouping [] option (0 or 1 times) {} repetition (0 to n times) ``` ## Query representation ### Characters InfluxQL is Unicode text encoded in [UTF-8](http://en.wikipedia.org/wiki/UTF-8). ``` newline = /* the Unicode code point U+000A */ . unicode_char = /* an arbitrary Unicode code point except newline */ . ``` ## Letters and digits Letters are the set of ASCII characters plus the underscore character _ (U+005F) is considered a letter. Only decimal digits are supported. ``` letter = ascii_letter | "_" . ascii_letter = "A" … "Z" | "a" … "z" . digit = "0" … "9" . ``` ## Identifiers Identifiers are tokens which refer to database names, retention policy names, user names, measurement names, tag keys, and field keys. The rules: - double quoted identifiers can contain any unicode character other than a new line - double quoted identifiers can contain escaped `"` characters (i.e., `\"`) - double quoted identifiers can contain InfluxQL keywords - unquoted identifiers must start with an upper or lowercase ASCII character or "_" - unquoted identifiers may contain only ASCII letters, decimal digits, and "_" ``` identifier = unquoted_identifier | quoted_identifier . unquoted_identifier = ( letter ) { letter | digit } . quoted_identifier = `"` unicode_char { unicode_char } `"` . ``` #### Examples: ``` cpu _cpu_stats "1h" "anything really" "1_Crazy-1337.identifier>NAME👍" ``` ## Keywords ``` ALL ALTER ANY AS ASC BEGIN BY CREATE CONTINUOUS DATABASE DATABASES DEFAULT DELETE DESC DESTINATIONS DIAGNOSTICS DISTINCT DROP DURATION END EVERY EXPLAIN FIELD FOR FROM GRANT GRANTS GROUP GROUPS IN INF INSERT INTO KEY KEYS KILL LIMIT SHOW MEASUREMENT MEASUREMENTS NAME OFFSET ON ORDER PASSWORD POLICY POLICIES PRIVILEGES QUERIES QUERY READ REPLICATION RESAMPLE RETENTION REVOKE SELECT SERIES SET SHARD SHARDS SLIMIT SOFFSET STATS SUBSCRIPTION SUBSCRIPTIONS TAG TO USER USERS VALUES WHERE WITH WRITE ``` ## Literals ### Integers InfluxQL supports decimal integer literals. Hexadecimal and octal literals are not currently supported. ``` int_lit = ( "1" … "9" ) { digit } . ``` ### Floats InfluxQL supports floating-point literals. Exponents are not currently supported. ``` float_lit = int_lit "." int_lit . ``` ### Strings String literals must be surrounded by single quotes. Strings may contain `'` characters as long as they are escaped (i.e., `\'`). ``` string_lit = `'` { unicode_char } `'` . ``` ### Durations Duration literals specify a length of time. An integer literal followed immediately (with no spaces) by a duration unit listed below is interpreted as a duration literal. ### Duration units | Units | Meaning | |--------|-----------------------------------------| | u or µ | microseconds (1 millionth of a second) | | ms | milliseconds (1 thousandth of a second) | | s | second | | m | minute | | h | hour | | d | day | | w | week | ``` duration_lit = int_lit duration_unit . duration_unit = "u" | "µ" | "ms" | "s" | "m" | "h" | "d" | "w" . ``` ### Dates & Times The date and time literal format is not specified in EBNF like the rest of this document. It is specified using Go's date / time parsing format, which is a reference date written in the format required by InfluxQL. The reference date time is: InfluxQL reference date time: January 2nd, 2006 at 3:04:05 PM ``` time_lit = "2006-01-02 15:04:05.999999" | "2006-01-02" . ``` ### Booleans ``` bool_lit = TRUE | FALSE . ``` ### Regular Expressions ``` regex_lit = "/" { unicode_char } "/" . ``` **Comparators:** `=~` matches against `!~` doesn't match against > **Note:** Use regular expressions to match measurements and tags. You cannot use regular expressions to match databases, retention policies, or fields. ## Queries A query is composed of one or more statements separated by a semicolon. ``` query = statement { ";" statement } . statement = alter_retention_policy_stmt | create_continuous_query_stmt | create_database_stmt | create_retention_policy_stmt | create_subscription_stmt | create_user_stmt | delete_stmt | drop_continuous_query_stmt | drop_database_stmt | drop_measurement_stmt | drop_retention_policy_stmt | drop_series_stmt | drop_shard_stmt | drop_subscription_stmt | drop_user_stmt | grant_stmt | kill_query_statement | show_continuous_queries_stmt | show_databases_stmt | show_field_keys_stmt | show_grants_stmt | show_measurements_stmt | show_queries_stmt | show_retention_policies | show_series_stmt | show_shard_groups_stmt | show_shards_stmt | show_subscriptions_stmt| show_tag_keys_stmt | show_tag_values_stmt | show_users_stmt | revoke_stmt | select_stmt . ``` ## Statements ### ALTER RETENTION POLICY ``` alter_retention_policy_stmt = "ALTER RETENTION POLICY" policy_name on_clause retention_policy_option [ retention_policy_option ] [ retention_policy_option ] [ retention_policy_option ] . ``` > Replication factors do not serve a purpose with single node instances. #### Examples: ```sql -- Set default retention policy for mydb to 1h.cpu. ALTER RETENTION POLICY "1h.cpu" ON "mydb" DEFAULT -- Change duration and replication factor. ALTER RETENTION POLICY "policy1" ON "somedb" DURATION 1h REPLICATION 4 ``` ### CREATE CONTINUOUS QUERY ``` create_continuous_query_stmt = "CREATE CONTINUOUS QUERY" query_name on_clause [ "RESAMPLE" resample_opts ] "BEGIN" select_stmt "END" . query_name = identifier . resample_opts = (every_stmt for_stmt | every_stmt | for_stmt) . every_stmt = "EVERY" duration_lit for_stmt = "FOR" duration_lit ``` #### Examples: ```sql -- selects from DEFAULT retention policy and writes into 6_months retention policy CREATE CONTINUOUS QUERY "10m_event_count" ON "db_name" BEGIN SELECT count("value") INTO "6_months"."events" FROM "events" GROUP BY time(10m) END; -- this selects from the output of one continuous query in one retention policy and outputs to another series in another retention policy CREATE CONTINUOUS QUERY "1h_event_count" ON "db_name" BEGIN SELECT sum("count") as "count" INTO "2_years"."events" FROM "6_months"."events" GROUP BY time(1h) END; -- this customizes the resample interval so the interval is queried every 10s and intervals are resampled until 2m after their start time -- when resample is used, at least one of "EVERY" or "FOR" must be used CREATE CONTINUOUS QUERY "cpu_mean" ON "db_name" RESAMPLE EVERY 10s FOR 2m BEGIN SELECT mean("value") INTO "cpu_mean" FROM "cpu" GROUP BY time(1m) END; ``` ### CREATE DATABASE ``` create_database_stmt = "CREATE DATABASE" db_name [ WITH [ retention_policy_duration ] [ retention_policy_replication ] [ retention_policy_shard_group_duration ] [ retention_policy_name ] ] . ``` > Replication factors do not serve a purpose with single node instances. #### Examples: ```sql -- Create a database called foo CREATE DATABASE "foo" -- Create a database called bar with a new DEFAULT retention policy and specify the duration, replication, shard group duration, and name of that retention policy CREATE DATABASE "bar" WITH DURATION 1d REPLICATION 1 SHARD DURATION 30m NAME "myrp" -- Create a database called mydb with a new DEFAULT retention policy and specify the name of that retention policy CREATE DATABASE "mydb" WITH NAME "myrp" ``` ### CREATE RETENTION POLICY ``` create_retention_policy_stmt = "CREATE RETENTION POLICY" policy_name on_clause retention_policy_duration retention_policy_replication [ retention_policy_shard_group_duration ] [ "DEFAULT" ] . ``` > Replication factors do not serve a purpose with single node instances. #### Examples ```sql -- Create a retention policy. CREATE RETENTION POLICY "10m.events" ON "somedb" DURATION 60m REPLICATION 2 -- Create a retention policy and set it as the DEFAULT. CREATE RETENTION POLICY "10m.events" ON "somedb" DURATION 60m REPLICATION 2 DEFAULT -- Create a retention policy and specify the shard group duration. CREATE RETENTION POLICY "10m.events" ON "somedb" DURATION 60m REPLICATION 2 SHARD DURATION 30m ``` ### CREATE SUBSCRIPTION Subscriptions tell InfluxDB to send all the data it receives to Kapacitor or other third parties. ``` create_subscription_stmt = "CREATE SUBSCRIPTION" subscription_name "ON" db_name "." retention_policy "DESTINATIONS" ("ANY"|"ALL") host { "," host} . ``` #### Examples: ```sql -- Create a SUBSCRIPTION on database 'mydb' and retention policy 'autogen' that send data to 'example.com:9090' via UDP. CREATE SUBSCRIPTION "sub0" ON "mydb"."autogen" DESTINATIONS ALL 'udp://example.com:9090' -- Create a SUBSCRIPTION on database 'mydb' and retention policy 'autogen' that round robins the data to 'h1.example.com:9090' and 'h2.example.com:9090'. CREATE SUBSCRIPTION "sub0" ON "mydb"."autogen" DESTINATIONS ANY 'udp://h1.example.com:9090', 'udp://h2.example.com:9090' ``` ### CREATE USER ``` create_user_stmt = "CREATE USER" user_name "WITH PASSWORD" password [ "WITH ALL PRIVILEGES" ] . ``` #### Examples: ```sql -- Create a normal database user. CREATE USER "jdoe" WITH PASSWORD '1337password' -- Create an admin user. -- Note: Unlike the GRANT statement, the "PRIVILEGES" keyword is required here. CREATE USER "jdoe" WITH PASSWORD '1337password' WITH ALL PRIVILEGES ``` > **Note:** The password string must be wrapped in single quotes. ### DELETE ``` delete_stmt = "DELETE" ( from_clause | where_clause | from_clause where_clause ) . ``` #### Examples: ```sql DELETE FROM "cpu" DELETE FROM "cpu" WHERE time < '2000-01-01T00:00:00Z' DELETE WHERE time < '2000-01-01T00:00:00Z' ``` ### DROP CONTINUOUS QUERY ``` drop_continuous_query_stmt = "DROP CONTINUOUS QUERY" query_name on_clause . ``` #### Example: ```sql DROP CONTINUOUS QUERY "myquery" ON "mydb" ``` ### DROP DATABASE ``` drop_database_stmt = "DROP DATABASE" db_name . ``` #### Example: ```sql DROP DATABASE "mydb" ``` ### DROP MEASUREMENT ``` drop_measurement_stmt = "DROP MEASUREMENT" measurement . ``` #### Examples: ```sql -- drop the cpu measurement DROP MEASUREMENT "cpu" ``` ### DROP RETENTION POLICY ``` drop_retention_policy_stmt = "DROP RETENTION POLICY" policy_name on_clause . ``` #### Example: ```sql -- drop the retention policy named 1h.cpu from mydb DROP RETENTION POLICY "1h.cpu" ON "mydb" ``` ### DROP SERIES ``` drop_series_stmt = "DROP SERIES" ( from_clause | where_clause | from_clause where_clause ) . ``` #### Example: ```sql DROP SERIES FROM "telegraf"."autogen"."cpu" WHERE cpu = 'cpu8' ``` ### DROP SHARD ``` drop_shard_stmt = "DROP SHARD" ( shard_id ) . ``` #### Example: ``` DROP SHARD 1 ``` ### DROP SUBSCRIPTION ``` drop_subscription_stmt = "DROP SUBSCRIPTION" subscription_name "ON" db_name "." retention_policy . ``` #### Example: ```sql DROP SUBSCRIPTION "sub0" ON "mydb"."autogen" ``` ### DROP USER ``` drop_user_stmt = "DROP USER" user_name . ``` #### Example: ```sql DROP USER "jdoe" ``` ### GRANT > **NOTE:** Users can be granted privileges on databases that do not exist. ``` grant_stmt = "GRANT" privilege [ on_clause ] to_clause . ``` #### Examples: ```sql -- grant admin privileges GRANT ALL TO "jdoe" -- grant read access to a database GRANT READ ON "mydb" TO "jdoe" ``` ### KILL QUERY ``` kill_query_statement = "KILL QUERY" query_id . ``` #### Examples: ``` --- kill a query with the query_id 36 KILL QUERY 36 ``` > **NOTE:** Identify the `query_id` from the `SHOW QUERIES` output. ### SHOW CONTINUOUS QUERIES ``` show_continuous_queries_stmt = "SHOW CONTINUOUS QUERIES" . ``` #### Example: ```sql -- show all continuous queries SHOW CONTINUOUS QUERIES ``` ### SHOW DATABASES ``` show_databases_stmt = "SHOW DATABASES" . ``` #### Example: ```sql -- show all databases SHOW DATABASES ``` ### SHOW FIELD KEYS ``` show_field_keys_stmt = "SHOW FIELD KEYS" [ from_clause ] . ``` #### Examples: ```sql -- show field keys and field value data types from all measurements SHOW FIELD KEYS -- show field keys and field value data types from specified measurement SHOW FIELD KEYS FROM "cpu" ``` ### SHOW GRANTS ``` show_grants_stmt = "SHOW GRANTS FOR" user_name . ``` #### Example: ```sql -- show grants for jdoe SHOW GRANTS FOR "jdoe" ``` ### SHOW MEASUREMENTS ``` show_measurements_stmt = "SHOW MEASUREMENTS" [ with_measurement_clause ] [ where_clause ] [ limit_clause ] [ offset_clause ] . ``` #### Examples: ```sql -- show all measurements SHOW MEASUREMENTS -- show measurements where region tag = 'uswest' AND host tag = 'serverA' SHOW MEASUREMENTS WHERE "region" = 'uswest' AND "host" = 'serverA' -- show measurements that start with 'h2o' SHOW MEASUREMENTS WITH MEASUREMENT =~ /h2o.*/ ``` ### SHOW QUERIES ``` show_queries_stmt = "SHOW QUERIES" . ``` #### Example: ```sql -- show all currently-running queries SHOW QUERIES ``` ### SHOW RETENTION POLICIES ``` show_retention_policies = "SHOW RETENTION POLICIES" on_clause . ``` #### Example: ```sql -- show all retention policies on a database SHOW RETENTION POLICIES ON "mydb" ``` ### SHOW SERIES ``` show_series_stmt = "SHOW SERIES" [ from_clause ] [ where_clause ] [ limit_clause ] [ offset_clause ] . ``` #### Example: ```sql SHOW SERIES FROM "telegraf"."autogen"."cpu" WHERE cpu = 'cpu8' ``` ### SHOW SHARD GROUPS ``` show_shard_groups_stmt = "SHOW SHARD GROUPS" . ``` #### Example: ```sql SHOW SHARD GROUPS ``` ### SHOW SHARDS ``` show_shards_stmt = "SHOW SHARDS" . ``` #### Example: ```sql SHOW SHARDS ``` ### SHOW SUBSCRIPTIONS ``` show_subscriptions_stmt = "SHOW SUBSCRIPTIONS" . ``` #### Example: ```sql SHOW SUBSCRIPTIONS ``` ### SHOW TAG KEYS ``` show_tag_keys_stmt = "SHOW TAG KEYS" [ from_clause ] [ where_clause ] [ group_by_clause ] [ limit_clause ] [ offset_clause ] . ``` #### Examples: ```sql -- show all tag keys SHOW TAG KEYS -- show all tag keys from the cpu measurement SHOW TAG KEYS FROM "cpu" -- show all tag keys from the cpu measurement where the region key = 'uswest' SHOW TAG KEYS FROM "cpu" WHERE "region" = 'uswest' -- show all tag keys where the host key = 'serverA' SHOW TAG KEYS WHERE "host" = 'serverA' ``` ### SHOW TAG VALUES ``` show_tag_values_stmt = "SHOW TAG VALUES" [ from_clause ] with_tag_clause [ where_clause ] [ group_by_clause ] [ limit_clause ] [ offset_clause ] . ``` #### Examples: ```sql -- show all tag values across all measurements for the region tag SHOW TAG VALUES WITH KEY = "region" -- show tag values from the cpu measurement for the region tag SHOW TAG VALUES FROM "cpu" WITH KEY = "region" -- show tag values across all measurements for all tag keys that do not include the letter c SHOW TAG VALUES WITH KEY !~ /.*c.*/ -- show tag values from the cpu measurement for region & host tag keys where service = 'redis' SHOW TAG VALUES FROM "cpu" WITH KEY IN ("region", "host") WHERE "service" = 'redis' ``` ### SHOW USERS ``` show_users_stmt = "SHOW USERS" . ``` #### Example: ```sql -- show all users SHOW USERS ``` ### REVOKE ``` revoke_stmt = "REVOKE" privilege [ on_clause ] "FROM" user_name . ``` #### Examples: ```sql -- revoke admin privileges from jdoe REVOKE ALL PRIVILEGES FROM "jdoe" -- revoke read privileges from jdoe on mydb REVOKE READ ON "mydb" FROM "jdoe" ``` ### SELECT ``` select_stmt = "SELECT" fields from_clause [ into_clause ] [ where_clause ] [ group_by_clause ] [ order_by_clause ] [ limit_clause ] [ offset_clause ] [ slimit_clause ] [ soffset_clause ] . ``` #### Examples: ```sql -- select mean value from the cpu measurement where region = 'uswest' grouped by 10 minute intervals SELECT mean("value") FROM "cpu" WHERE "region" = 'uswest' GROUP BY time(10m) fill(0) -- select from all measurements beginning with cpu into the same measurement name in the cpu_1h retention policy SELECT mean("value") INTO "cpu_1h".:MEASUREMENT FROM /cpu.*/ ``` ## Clauses ``` from_clause = "FROM" measurements . group_by_clause = "GROUP BY" dimensions fill(fill_option). into_clause = "INTO" ( measurement | back_ref ). limit_clause = "LIMIT" int_lit . offset_clause = "OFFSET" int_lit . slimit_clause = "SLIMIT" int_lit . soffset_clause = "SOFFSET" int_lit . on_clause = "ON" db_name . order_by_clause = "ORDER BY" sort_fields . to_clause = "TO" user_name . where_clause = "WHERE" expr . with_measurement_clause = "WITH MEASUREMENT" ( "=" measurement | "=~" regex_lit ) . with_tag_clause = "WITH KEY" ( "=" tag_key | "!=" tag_key | "=~" regex_lit | "IN (" tag_keys ")" ) . ``` ## Expressions ``` binary_op = "+" | "-" | "*" | "/" | "AND" | "OR" | "=" | "!=" | "<>" | "<" | "<=" | ">" | ">=" . expr = unary_expr { binary_op unary_expr } . unary_expr = "(" expr ")" | var_ref | time_lit | string_lit | int_lit | float_lit | bool_lit | duration_lit | regex_lit . ``` ## Other ``` alias = "AS" identifier . back_ref = ( policy_name ".:MEASUREMENT" ) | ( db_name "." [ policy_name ] ".:MEASUREMENT" ) . db_name = identifier . dimension = expr . dimensions = dimension { "," dimension } . field_key = identifier . field = expr [ alias ] . fields = field { "," field } . fill_option = "null" | "none" | "previous" | "linear" | int_lit | float_lit . host = string_lit . measurement = measurement_name | ( policy_name "." measurement_name ) | ( db_name "." [ policy_name ] "." measurement_name ) . measurements = measurement { "," measurement } . measurement_name = identifier | regex_lit . password = string_lit . policy_name = identifier . privilege = "ALL" [ "PRIVILEGES" ] | "READ" | "WRITE" . query_id = int_lit . query_name = identifier . retention_policy = identifier . retention_policy_option = retention_policy_duration | retention_policy_replication | retention_policy_shard_group_duration | "DEFAULT" . retention_policy_duration = "DURATION" duration_lit . retention_policy_replication = "REPLICATION" int_lit . retention_policy_shard_group_duration = "SHARD DURATION" duration_lit . retention_policy_name = "NAME" identifier . series_id = int_lit . shard_id = int_lit . sort_field = field_key [ ASC | DESC ] . sort_fields = sort_field { "," sort_field } . subscription_name = identifier . tag_key = identifier . tag_keys = tag_key { "," tag_key } . user_name = identifier . var_ref = measurement . ``` ## Query Engine Internals Once you understand the language itself, it's important to know how these language constructs are implemented in the query engine. This gives you an intuitive sense for how results will be processed and how to create efficient queries. The life cycle of a query looks like this: 1. InfluxQL query string is tokenized and then parsed into an abstract syntax tree (AST). This is the code representation of the query itself. 2. The AST is passed to the `QueryExecutor` which directs queries to the appropriate handlers. For example, queries related to meta data are executed by the meta service and `SELECT` statements are executed by the shards themselves. 3. The query engine then determines the shards that match the `SELECT` statement's time range. From these shards, iterators are created for each field in the statement. 4. Iterators are passed to the emitter which drains them and joins the resulting points. The emitter's job is to convert simple time/value points into the more complex result objects that are returned to the client. ### Understanding Iterators Iterators are at the heart of the query engine. They provide a simple interface for looping over a set of points. For example, this is an iterator over Float points: ``` type FloatIterator interface { Next() *FloatPoint } ``` These iterators are created through the `IteratorCreator` interface: ``` type IteratorCreator interface { CreateIterator(opt *IteratorOptions) (Iterator, error) } ``` The `IteratorOptions` provide arguments about field selection, time ranges, and dimensions that the iterator creator can use when planning an iterator. The `IteratorCreator` interface is used at many levels such as the `Shards`, `Shard`, and `Engine`. This allows optimizations to be performed when applicable such as returning a precomputed `COUNT()`. Iterators aren't just for reading raw data from storage though. Iterators can be composed so that they provided additional functionality around an input iterator. For example, a `DistinctIterator` can compute the distinct values for each time window for an input iterator. Or a `FillIterator` can generate additional points that are missing from an input iterator. This composition also lends itself well to aggregation. For example, a statement such as this: ``` SELECT MEAN(value) FROM cpu GROUP BY time(10m) ``` In this case, `MEAN(value)` is a `MeanIterator` wrapping an iterator from the underlying shards. However, if we can add an additional iterator to determine the derivative of the mean: ``` SELECT DERIVATIVE(MEAN(value), 20m) FROM cpu GROUP BY time(10m) ``` ### Understanding Auxiliary Fields Because InfluxQL allows users to use selector functions such as `FIRST()`, `LAST()`, `MIN()`, and `MAX()`, the engine must provide a way to return related data at the same time with the selected point. For example, in this query: ``` SELECT FIRST(value), host FROM cpu GROUP BY time(1h) ``` We are selecting the first `value` that occurs every hour but we also want to retrieve the `host` associated with that point. Since the `Point` types only specify a single typed `Value` for efficiency, we push the `host` into the auxiliary fields of the point. These auxiliary fields are attached to the point until it is passed to the emitter where the fields get split off to their own iterator. ### Built-in Iterators There are many helper iterators that let us build queries: * Merge Iterator - This iterator combines one or more iterators into a single new iterator of the same type. This iterator guarantees that all points within a window will be output before starting the next window but does not provide ordering guarantees within the window. This allows for fast access for aggregate queries which do not need stronger sorting guarantees. * Sorted Merge Iterator - This iterator also combines one or more iterators into a new iterator of the same type. However, this iterator guarantees time ordering of every point. This makes it slower than the `MergeIterator` but this ordering guarantee is required for non-aggregate queries which return the raw data points. * Limit Iterator - This iterator limits the number of points per name/tag group. This is the implementation of the `LIMIT` & `OFFSET` syntax. * Fill Iterator - This iterator injects extra points if they are missing from the input iterator. It can provide `null` points, points with the previous value, or points with a specific value. * Buffered Iterator - This iterator provides the ability to "unread" a point back onto a buffer so it can be read again next time. This is used extensively to provide lookahead for windowing. * Reduce Iterator - This iterator calls a reduction function for each point in a window. When the window is complete then all points for that window are output. This is used for simple aggregate functions such as `COUNT()`. * Reduce Slice Iterator - This iterator collects all points for a window first and then passes them all to a reduction function at once. The results are returned from the iterator. This is used for aggregate functions such as `DERIVATIVE()`. * Transform Iterator - This iterator calls a transform function for each point from an input iterator. This is used for executing binary expressions. * Dedupe Iterator - This iterator only outputs unique points. It is resource intensive so it is only used for small queries such as meta query statements. ### Call Iterators Function calls in InfluxQL are implemented at two levels. Some calls can be wrapped at multiple layers to improve efficiency. For example, a `COUNT()` can be performed at the shard level and then multiple `CountIterator`s can be wrapped with another `CountIterator` to compute the count of all shards. These iterators can be created using `NewCallIterator()`. Some iterators are more complex or need to be implemented at a higher level. For example, the `DERIVATIVE()` needs to retrieve all points for a window first before performing the calculation. This iterator is created by the engine itself and is never requested to be created by the lower levels.