cloud-fan opened a new pull request #30412:
URL: https://github.com/apache/spark/pull/30412
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### What changes were proposed in this pull request?
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This PR adds the char/varchar type which is kind of a variant of string type:
1. Char type is fixed-length string. When comparing char type values, we
need to pad the shorter one to the longer length.
2. Varchar type is string with a length limitation.
To implement the char/varchar semantic, this PR:
1. Do string length check when writing to char/varchar type columns.
2. Do string padding when reading char type columns. We don't do it at the
writing side to save storage space.
3. Do string padding when comparing char type column with string literal or
another char type column. (string literal is fixed length so should be treated
as char type as well)
To simplify the implementation, this PR doesn't propagate char/varchar type
info through functions/operators(e.g. `substring`). That said, a column can
only be char/varchar type if it's a table column, not a derived column like
`SELECT substring(col)`.
To be safe, this PR doesn't add char/varchar type to the core type system.
We will replace char/varchar type by string type with metadata
(`Attribute.metadata` or `StructField.metadata`) that includes the original
type string before it goes into the query engine. That said, the existing code
will not see char/varchar type but only string type.
char/varchar type may come from several places:
1. v1 table from hive catalog.
2. v2 table from v2 catalog.
3. user-specified schema in `spark.read.schema` and `spark.readStream.schema`
4. `Column.cast`
This PR covers all the above cases, implements the length check and padding
feature by looking at string type with special metadata.
### Why are the changes needed?
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char and varchar are standard SQL types. varchar is widely used in other
databases instead of string type.
### Does this PR introduce _any_ user-facing change?
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For hive tables: now the table insertion fails if the value exceeds
char/varchar length. Previously we truncate the value silently.
For other tables:
1. now char type is allowed.
2. now we have length check when inserting to varchar columns. Previously we
write the value as it is.
### How was this patch tested?
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new tests
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