Add function key expressions for computing record size and ternary expressions#4163
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…pressions This adds a few new useful function key expressions to allow users to specify more complicated ideas through key expressions. The first is a record size function key expression. It behaves a bit like the `VersionKeyExpression` in that it doesn't really take arguments, and it returns a value (the serialized size) that comes from the `FDBRecord` rather than the non-record inputs into the expression. The second is a ternary function key expression. It allows the user to define expressions like `a.bool_field ? <expr1> : <expr2>`. It would prepare us for some what more complicated expressions like `a.x < 10 ? <expr1> : <expr2>`. You could also theoretically implement a coaelsce function on top of it (`x IS NULL ? y : x` for `coalesce(x, y)`), but you are limited by the set of expressions that could actually be Booleans.
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This adds a few new useful function key expressions to allow users to specify more complicated ideas through key expressions.
The first is a record size function key expression. It behaves a bit like the
VersionKeyExpressionin that it doesn't really take arguments, and it returns a value (the serialized size) that comes from theFDBRecordrather than the non-record inputs into the expression.The second is a ternary function key expression. It allows the user to define expressions like
a.bool_field ? <expr1> : <expr2>. It would prepare us for some what more complicated expressions likea.x < 10 ? <expr1> : <expr2>. You could also theoretically implement a coaelsce function on top of it (x IS NULL ? y : xforcoalesce(x, y)), but you are limited by the set of expressions that could actually be Booleans.