feature: Pass the value of the expression through parameters

[xiaocai2333]

issue: #milvus-io/milvus#36672

[liliu-z]

Plz CMIIW, in #37033 we use the following code to read the value out of this FieldData

result[data.GetFieldName()] = rv[0]

And use value, ok := data[expr.GetPlaceholderName()] to fill the expr. And result above equals to data here.

So our assumption is that the placeholder name is the field name?

Also, can you provide some examples showing how we will fill this expression_values?

[zhengbuqian]

I think here FieldData is only used as a container to hold the template variable name: value mapping.

[zhengbuqian]

I wonder is schema.FieldData the best container to use? schema.PlaceholderValue seems better.

[zhengbuqian]

when using schema.FieldData we assigned new meaning to FieldData::field_name

[zhengbuqian]

chatted with @czs007 offline:

• the main reason we are using schema.FieldData is to utilize the Base 128 variant encoding capability to have some levels of compression on int fields.
  • this encoding doesn't apply to floating types
  • it also doesn't apply to string types
  • Technically Base 128 variant is NOT a compression algorithm. Protobuf does not have built-in compression.
• using fieldName member of schema.FieldData as the template variable name is indeed confusing. And it will be painful if we want to modify this due to the need of backward compatibility.

maybe the best option is to define a new proto message similar to FieldData but with correct member naming.

[zhengbuqian]

defining a new proto message also reduces complexity in impl. we need only a single value whereas schema.FieldData is a repeated, currently in convert_field_data_to_generic_value.go we consistently loop through the repeated field and then assert its size to be 1. In the new proto we can directly define it as not repeated.

[yhmo]

/lgtm
/approve

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