feat: add COALESCE function #4829
Conversation
The `COALESCE` function takes one or more parameters and returns the first that is non-null. All parameters must be of the same type. This function supersedes the existing `IFNULL` function, which only worked for strings and one accepted two parameters.
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@blueedgenick @rmoff @derekjn @MichaelDrogalis: Just checking from an SQL / Product view that we're cool with this going in and deprecating |
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LGTM, thanks Andy! |
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Raised #4831 to track the removal of the deprecated IFNULL pre version 1.0. Alternatively, we could upgrade IFNULL to support any type and keep it, but it is basically a subset of COALESCE... |
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LGTM, solid addition 👍 |
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Thanks @big-andy-coates , appreciate you soliciting the input. The new COALESCE looks good to me, could benefit from describing in the docs it's behavior with the "non-traditional" i.e. complex datatypes (map, array, struct) - e.g. is a struct containing a null considered to be a null value or not for the purposes of this comparison ? (i'd assume and hope that it's not, but worth calling out and maybe adding tests for). Likewise for an array with a single null entry and so on. These are the situations that may be unfamiliar to a user who is otherwise experienced with using a COALESCE function. Not keen on removing the IfNull though, i'd much prefer to fix it to handle all types. I know it looks like a subset of COALESCE but it's common in some SQL dialects where COALESCE is not present - it's ok i think to have slightly overlapping functions if they help ease the mental model for users coming from different flavors of RDBMS background. At least MySQL and Vertica, by way of example, have both functions. SQLServer has ISNULL and COALESCE, etc. One subtle difference to notice actually is that in many of the common DBMS, COALESCE is actually sugar for a CASE WHEN....CASE WHEN... construct. This is (slightly) significant because it's a "searched case" - meaning that each argument is only evaluated if needed, i.e. if the preceding ones turned out to be null. IFNULL, by contrast, always evaluates both input expressions. Our COALESCE implementation works in a more naive way, having first evaluated all of the expressions, put them in an array, and then passing to the UDF. This is slightly less efficient, especially if the parameters are themselves nested function calls. Not sure if we care about that just yet but it's worth noting for later. |
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I've removed the deprecation, updated the docs and converted #4831 into a task to enhance IFNULL to support more than just STRING args. |
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All good points @blueedgenick, thanks for chiming in 👍 |
Thanks! LGTM |
Moved to separate PR.
Description
The
COALESCEfunction takes one or more parameters and returns the first that is non-null.All parameters must be of the same type.
This function supersedes the existing
IFNULLfunction, which only worked for strings and one accepted two parameters.Note: also added docs for the
ARRAY_LENGHI added recently, as I seemed to of missed adding docs!Testing done
QTT + unit tests added.
Reviewer checklist