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Add missing documentation for constant post-aggregator (#13664) 

Thanks @anshu-makkar , I was waiting for CI to complete yesterday. Failures seem unrelated, so merging.
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Anshu Makkar 2023-02-09 22:23:45 +05:30 committed by GitHub
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docs/development/extensions-core/datasketches-theta.md
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@ -95,6 +95,49 @@ This returns a summary of the sketch that can be used for debugging. This is the
}
```
### Constant Theta Sketch
You can use the constant theta sketch post aggregator to add a Base64-encoded constant theta sketch value for use in other post-aggregators. For example, `thetaSketchSetOp`.
```json
{
"type" : "thetaSketchConstant",
"name": DESTINATION_COLUMN_NAME,
"value" : CONSTANT_SKETCH_VALUE
}
```
### Example using a constant Theta Sketch
Assume you have a datasource with a variety of a variety of users. Using `filters` and `aggregation`, you generate a theta sketch of all `football fans`.
A third-party provider has provided a constant theta sketch of all `cricket fans` and you want to `INTERSECT` both cricket fans and football fans in a `post-aggregation` stage to identify users who are interested in both `cricket`. Then you want to use `thetaSketchEstimate` to calculate the number of unique users.
```json
{
"type":"thetaSketchEstimate",
"name":"football_cricket_users_count",
"field":{
"type":"thetaSketchSetOp",
"name":"football_cricket_fans_users_theta_sketch",
"func":"INTERSECT",
"fields":[
{
"type":"fieldAccess",
"fieldName":"football_fans_users_theta_sketch"
},
{
"type":"thetaSketchConstant",
"name":"cricket_fans_users_theta_sketch",
"value":"AgMDAAAazJMCAAAAAACAPzz9j7pWTMdROWGf15uY1nI="
}
]
}
}
```
## Examples
Assuming, you have a dataset containing (timestamp, product, user_id). You want to answer questions like