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lookup operator

Extends the columns of a fact table with values looked-up in a dimension table.

FactTable | lookup kind=leftouter (DimensionTable) on CommonColumn, $left.Col1 == $right.Col2

Here, the result is a table that extends the FactTable ($left) with data from DimensionTable (referenced by $right) by performing a lookup of each pair (CommonColumn,Col) from the former table with each pair (CommonColumn1,Col2) in the latter table. For the differences between fact and dimension tables, see fact and dimension tables.

The lookup operator performs an operation similar to the join operator with the following differences:

  • The result does not repeat columns from the $right table that are the basis for the join operation.
  • Only two kinds of lookup are supported, leftouter and inner, with leftouter being the default.
  • In terms of performance, the system by default assumes that the $left table is the larger (facts) table, and the $right table is the smaller (dimensions) table. This is exactly opposite to the assumption used by the join operator.
  • The lookup operator automatically broadcasts the $right table to the $left table (essentially, behaves as if hint.broadcast was specified). Note that this limits the size of the $right table.


If the right side of the lookup is larger than several tens of MBs, the query will fail.

You can run the following query to estimate the size of the right side in bytes:

| summarize sum(estimate_data_size(*))


LeftTable | lookup [kind = (leftouter|inner)] ( RightTable ) on Attributes


  • LeftTable: The table or tabular expression that is the basis for the lookup. Denoted as $left.

  • RightTable: The table or tabular expression that is used to "populate" new columns in the fact table. Denoted as $right.

  • Attributes: A comma-delimited list of one or more rules that describe how rows from LeftTable are matched to rows from RightTable. Multiple rules are evaluated using the and logical operator. A rule can be one of:

Rule kind Syntax Predicate
Equality by name ColumnName where LeftTable.ColumnName == RightTable.ColumnName
Equality by value $left.LeftColumn == $right.RightColumn where $left.LeftColumn == $right.*RightColumn


In case of 'equality by value', the column names must be qualified with the applicable owner table denoted by $left and $right notations.

  • kind: An optional instruction on how to treat rows in LeftTable that have no match in RightTable. By default, leftouter is used, which means all those rows will appear in the output with null values used for the missing values of RightTable columns added by the operator. If inner is used, such rows are omitted from the output. (Other kinds of join are not supported by the lookup operator.)


A table with:

  • A column for every column in each of the two tables, including the matching keys. The columns of the right side will be automatically renamed if there are name conflicts.

  • A row for every match between the input tables. A match is a row selected from one table that has the same value for all the on fields as a row in the other table.

  • The Attributes (lookup keys) will appear only once in the output table.

  • kind unspecified, kind=leftouter

    In addition to the inner matches, there's a row for every row on the left (and/or right), even if it has no match. In that case, the unmatched output cells contain nulls.

  • kind=inner

    There's a row in the output for every combination of matching rows from left and right.


    | project
    | where original_time > ago(24h) and isnotempty(data_source_id)
    | lookup datasources on $left.data_source_id == $right.data_source_id
    | summarize EventCount=count_distinct(event_uuid) by DataSourceName=data_source_name
    | order by EventCount desc
    | take 10


DataSourceName EventCount
Cisco ACE Firewall@ 112346
test-UseCase0lookupDataSource 9123
MicroStWindows@Test3 123