Affects Version/s: None
Fix Version/s: None
The Union type is an "experimental" type that has never been completed. Yet, we use it as if it works.
Consider the test TestTopNSchemaChanges.testMissingColumn(). Run this with the new batch validator enabled. This test creates a union vector. Here is how the schema looks:
This is very hard to follow because the Union vector structure is complex (and has many issues.) Let's work though it.
We are looking at the MaterializedField for the union vector. It tells us that this Union has two types: FLOAT8 and INT. All good.
The Union has a vector per type, stored in an "internal map".' That map shows up as child, it is there on the children list as internal. However, the metadata claims that only one vector exists in that map: the types vector (the one that tells us what type to use for each row.) The vectors for FLOAT8 and INT are missing.
If, however, we use our debugger and inspect the actual contents of the internal map, we get the following:
That is, the internal map has the correct schema, but the Union vector itself has the wrong (incomplete) schema.
This is an inherent design flaw with Union vector: it requires two copies of the schema to be in sync. Further MaterializedField was designed to be immutable, but the map and Union types require mutation. If the Union simply points to the actual Map vector MaterializedField, it will drift out of date since the map vector creates a new schema each time we add fields; the Union vector ends up pointing to the old one.
This is not a simple bug to fix, but the result of the bug is that the vectors end up corrupted, as detected by the Batch Validator. In fact, the bug itself is subtle.
The TopNBatch does pass vector validation. However, because of the incorrect metadata, the downstream RemovingRecordBatch creates the derived Union vector incorrectly: it fails to set the value count for the INT type.
Where kl-type-INT is an ad-hoc way of saying we are checking the INT type vector for a Union named kl.
The schema of Union out of the RemovingRecordBatch has been truncated. The Union itself:
The internal map:
Notice that the FLOAT8 vector has disappeared: the Union vector metadata claims we have such a vector, but the internal map does not actually contain the vector.
The root cause is that the vector checker (indeed, any client) will call UnionVector.getMember(type) to get a vector for a type. This method includes a switch statement to call, say, getIntVector(). That method, in turn, creates the vector if does not exist.
But, since we are reading, we have an existing data batch. When we create a new vector, we create it as zero size. Thus, we think we have n records (three in this case), but we actually have zero. This kinda-sorta works because the type vector won't ever contain an entry for the "runt" vector, so we won't actually access data. But, this is an inconsistent structure. It breaks if we peer inside, as we are doing in the batch validator.
If we check for this case, we now get:
This is why Union is such a mess: is this a bug or just a very fragile design? I claim bug.