emkornfield commented on code in PR #461:
URL: https://github.com/apache/parquet-format/pull/461#discussion_r1881045255
##########
VariantShredding.md:
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@@ -25,276 +25,302 @@
The Variant type is designed to store and process semi-structured data
efficiently, even with heterogeneous values.
Query engines encode each Variant value in a self-describing format, and store
it as a group containing `value` and `metadata` binary fields in Parquet.
Since data is often partially homogenous, it can be beneficial to extract
certain fields into separate Parquet columns to further improve performance.
-We refer to this process as **shredding**.
-Each Parquet file remains fully self-describing, with no additional metadata
required to read or fully reconstruct the Variant data from the file.
-Combining shredding with a binary residual provides the flexibility to
represent complex, evolving data with an unbounded number of unique fields
while limiting the size of file schemas, and retaining the performance benefits
of a columnar format.
+This process is **shredding**.
-This document focuses on the shredding semantics, Parquet representation,
implications for readers and writers, as well as the Variant reconstruction.
-For now, it does not discuss which fields to shred, user-facing API changes,
or any engine-specific considerations like how to use shredded columns.
-The approach builds upon the [Variant Binary Encoding](VariantEncoding.md),
and leverages the existing Parquet specification.
+Shredding enables the use of of Parquet's columnar representation for more
compact data encoding, the use of column statistics for data skipping, and
partial projections from Parquet's columnar layout.
-At a high level, we replace the `value` field of the Variant Parquet group
with one or more fields called `object`, `array`, `typed_value`, and
`variant_value`.
-These represent a fixed schema suitable for constructing the full Variant
value for each row.
+For example, the query `SELECT variant_get(event, '$.event_ts', 'timestamp')
FROM tbl` only needs to load field `event_ts`, and shredding can enable
columnar projection that ignores the rest of the `event` Variant.
+Similarly, for the query `SELECT * FROM tbl WHERE variant_get(event,
'$.event_type', 'string') = 'signup'`, the `event_type` shredded column
metadata can be used for skipping and to lazily load the rest of the Variant.
-Shredding allows a query engine to reap the full benefits of Parquet's
columnar representation, such as more compact data encoding, min/max statistics
for data skipping, and I/O and CPU savings from pruning unnecessary fields not
accessed by a query (including the non-shredded Variant binary data).
-Without shredding, any query that accesses a Variant column must fetch all
bytes of the full binary buffer.
-With shredding, we can get nearly equivalent performance as in a relational
(scalar) data model.
+## Variant Metadata
-For example, `select variant_get(variant_col, ‘$.field1.inner_field2’,
‘string’) from tbl` only needs to access `inner_field2`, and the file scan
could avoid fetching the rest of the Variant value if this field was shredded
into a separate column in the Parquet schema.
-Similarly, for the query `select * from tbl where variant_get(variant_col,
‘$.id’, ‘integer’) = 123`, the scan could first decode the shredded `id`
column, and only fetch/decode the full Variant value for rows that pass the
filter.
+Variant metadata is stored in the top-level Variant group in a binary
`metadata` column regardless of whether the Variant value is shredded.
-# Parquet Example
+All `value` columns within the Variant must use the same `metadata`.
+All field names of a Variant, whether shredded or not, must be present in the
metadata.
-Consider the following Parquet schema together with how Variant values might
be mapped to it.
-Notice that we represent each shredded field in `object` as a group of two
fields, `typed_value` and `variant_value`.
-We extract all homogenous data items of a certain path into `typed_value`, and
set aside incompatible data items in `variant_value`.
-Intuitively, incompatibilities within the same path may occur because we store
the shredding schema per Parquet file, and each file can contain several row
groups.
-Selecting a type for each field that is acceptable for all rows would be
impractical because it would require buffering the contents of an entire file
before writing.
+## Value Shredding
-Typically, the expectation is that `variant_value` exists at every level as an
option, along with one of `object`, `array` or `typed_value`.
-If the actual Variant value contains a type that does not match the provided
schema, it is stored in `variant_value`.
-An `variant_value` may also be populated if an object can be partially
represented: any fields that are present in the schema must be written to those
fields, and any missing fields are written to `variant_value`.
-
-The `metadata` column is unchanged from its unshredded representation, and may
be referenced in `variant_value` fields in the shredded data.
+Variant values are stored in Parquet fields named `value`.
+Each `value` field may have an associated shredded field named `typed_value`
that stores the value when it matches a specific type.
+For example, a Variant field, `measurement` may be shredded as long values by
adding `typed_value` with type `int64`:
```
-optional group variant_col {
- required binary metadata;
- optional binary variant_value;
- optional group object {
- optional group a {
- optional binary variant_value;
- optional int64 typed_value;
- }
- optional group b {
- optional binary variant_value;
- optional group object {
- optional group c {
- optional binary variant_value;
- optional binary typed_value (STRING);
- }
- }
- }
- }
+required group measurement (VARIANT) {
+ required binary metadata;
+ optional binary value;
+ optional int64 typed_value;
}
```
-| Variant Value | Top-level variant_value | b.variant_value | a.typed_value |
a.variant_value | b.object.c.typed_value | b.object.c.variant_value | Notes |
-|---------------|-------------------------|-----------------|---------------|-----------------|------------------------|--------------------------|-------|
-| {a: 123, b: {c: “hello”}} | null | null | 123 | null | hello | null | All
values shredded |
-| {a: 1.23, b: {c: “123”}} | null | null | null | 1.23 | 123 | null | a is not
an integer |
-| {a: 123, b: {c: null}} | null | null | null | 123 | null | null | b.object.c
set to non-null to indicate VariantNull |
-| {a: 123, b: {} | null | null | null | 123 | null | null | b.object.c set to
null, to indicate that c is missing |
-| {a: 123, d: 456} | {d: 456} | null | 123 | null | null | null | Extra field
d is stored as variant_value |
-| [{a: 1, b: {c: 2}}, {a: 3, b: {c: 4}}] | [{a: 1, b: {c: 2}}, {a: 3, b: {c:
4}}] | null | null | null | null | null | Not an object |
+The series of measurements `34, null, "n/a", 100` would be stored as:
-# Parquet Layout
+| Value | `metadata` | `value` | `typed_value` |
+|---------|------------------|-----------------------|---------------|
+| 34 | `01 00` v1/empty | null | `34` |
+| null | `01 00` v1/empty | `00` (null) | null |
+| "n/a" | `01 00` v1/empty | `13 6E 2F 61` (`n/a`) | null |
+| 100 | `01 00` v1/empty | null | `100` |
-The `array` and `object` fields represent Variant array and object types,
respectively.
-Arrays must use the three-level list structure described in
https://github.com/apache/parquet-format/blob/master/LogicalTypes.md.
+Both `value` and `typed_value` are optional fields used together to encode a
single value.
+Values in the two fields must be interpreted according to the following table:
-An `object` field must be a group.
-Each field name of this inner group corresponds to the Variant value's object
field name.
-Each inner field's type is a recursively shredded variant value: that is, the
fields of each object field must be one or more of `object`, `array`,
`typed_value` or `variant_value`.
+| `value` | `typed_value` | Meaning
|
+|----------|---------------|----------------------------------------------------------|
+| null | null | The value is missing
|
+| non-null | null | The value is present and may be any type,
including null |
+| null | non-null | The value is present and is the shredded type
|
+| non-null | non-null | The value is present and is a partially shredded
object |
-Similarly the elements of an `array` must be a group containing one or more of
`object`, `array`, `typed_value` or `variant_value`.
+An object is _partially shredded_ when the `value` is an object and the
`typed_value` is a shredded object.
-Each leaf in the schema can store an arbitrary Variant value.
-It contains an `variant_value` binary field and a `typed_value` field.
-If non-null, `variant_value` represents the value stored as a Variant binary.
-The `typed_value` field may be any type that has a corresponding Variant type.
-For each value in the data, at most one of the `typed_value` and
`variant_value` may be non-null.
-A writer may omit either field, which is equivalent to all rows being null.
+If both fields are non-null and either is not an object, the value is invalid.
Readers must either fail or return the `typed_value`.
-Dictionary IDs in a `variant_value` field refer to entries in the top-level
`metadata` field.
+If a Variant is missing in a context where a value is required, readers must
either fail or return a Variant null: basic type 0 (primitive) and physical
type 0 (null).
+For example, if a Variant is required (like `measurement` above) and both
`value` and `typed_value` are null, the returned `value` must be `00` (Variant
null).
-For an `object`, a null field means that the field does not exist in the
reconstructed Variant object.
-All elements of an `array` must be non-null, since array elements cannote be
missing.
+### Shredded Value Types
-| typed_value | variant_value | Meaning |
-|-------------|----------------|---------|
-| null | null | Field is Variant Null (not missing) in the reconstructed
Variant. |
-| null | non-null | Field may be any type in the reconstructed Variant. |
-| non-null | null | Field has this column’s type in the reconstructed Variant.
|
-| non-null | non-null | Invalid |
+Shredded values must use the following Parquet types:
-The `typed_value` may be absent from the Parquet schema for any field, which
is equivalent to its value being always null (in which case the shredded field
is always stored as a Variant binary).
-By the same token, `variant_value` may be absent, which is equivalent to their
value being always null (in which case the field will always have the value
Null or have the type of the `typed_value` column).
+| Variant Type | Equivalent Parquet Type |
+|-----------------------------|------------------------------|
+| boolean | BOOLEAN |
+| int8 | INT(8, signed=true) |
+| int16 | INT(16, signed=true) |
+| int32 | INT32 / INT(32, signed=true) |
+| int64 | INT64 / INT(64, signed=true) |
+| float | FLOAT |
+| double | DOUBLE |
+| decimal4 | DECIMAL(precision, scale) |
+| decimal8 | DECIMAL(precision, scale) |
+| decimal16 | DECIMAL(precision, scale) |
+| date | DATE |
+| timestamp | TIMESTAMP(true, MICROS) |
+| timestamp without time zone | TIMESTAMP(false, MICROS) |
+| binary | BINARY |
+| string | STRING |
+| array | LIST; see Arrays below |
+| object | GROUP; see Objects below |
-# Unshredded values
+#### Primitive Types
-If all values can be represented at a given level by whichever of `object`,
`array`, or `typed_value` is present, `variant_value` is set to null.
+Primitive values can be shredded using the equivalent Parquet primitive type
from the table above for `typed_object`.
-If a value cannot be represented by whichever of `object`, `array`, or
`typed_value` is present in the schema, then it is stored in `variant_value`,
and the other fields are set to null.
-In the Parquet example above, if field `a` was an object or array, or a
non-integer scalar, it would be stored in `variant_value`.
+Unless the value is shredded as an object (see [Objects](#objects)),
`typed_value` or `value` (but not both) must be non-null.
-If a value is an object, and the `object` field is present but does not
contain all of the fields in the value, then any remaining fields are stored in
an object in `variant_value`.
-In the Parquet example above, if field `b` was an object of the form `{"c": 1,
"d": 2}"`, then the object `{"d": 2}` would be stored in `variant_value`, and
the `c` field would be shredded recursively under `object.c`.
+#### Arrays
-Note that an array is always fully shredded if there is an `array` field, so
the above consideration for `object` is not relevant for arrays: only one of
`array` or `variant_value` may be non-null at a given level.
+Arrays can be shredded using a 3-level Parquet list for `typed_value`.
-# Using variant_value vs. typed_value
+If the value is not an array, `typed_value` must be null.
+If the value is an array, `value` must be null.
-In general, it is desirable to store values in the `typed_value` field rather
than the `variant_value` whenever possible.
-This will typically improve encoding efficiency, and allow the use of Parquet
statistics to filter at the row group or page level.
-In the best case, the `variant_value` fields are all null and the engine does
not need to read them (or it can omit them from the schema on write entirely).
-There are two main motivations for including the `variant_value` column:
+The list `element` must be a required group that contains optional `value` and
`typed_value` fields.
+The element's `value` field stores the element as Variant-encoded `binary`
when the `typed_value` cannot represent it.
-1) In a case where there are rare type mismatches (for example, a numeric
field with rare strings like “n/a”), we allow the field to be shredded, which
could still be a significant performance benefit compared to fetching and
decoding the full value/metadata binary.
-2) Since there is a single schema per file, there would be no easy way to
recover from a type mismatch encountered late in a file write. Parquet files
can be large, and buffering all file data before starting to write could be
expensive. Including a variant column for every field guarantees we can adhere
to the requested shredding schema.
+For example, a `tags` Variant may be shredded as a list of strings using the
following definition:
+```
+optional group tags (VARIANT) {
+ required binary metadata;
+ optional binary value;
+ optional group typed_value (LIST) { # must be optional to allow a null list
+ repeated group list {
+ required group element { # shredded element
+ optional binary value;
+ optional binary typed_value (STRING);
+ }
+ }
+ }
+}
+```
-# Data Skipping
+All elements of an array must be non-null because `array` elements in a
Variant cannot be missing.
+That is, either `typed_value` or `value` (but not both) must be non-null.
+Null elements must be encoded in `value` as Variant null: basic type 0
(primitive) and physical type 0 (null).
-Shredded columns are expected to store statistics in the same format as a
normal Parquet column.
-In general, the engine can only skip a row group or page if all rows in the
`variant_value` field are null, since it is possible for a `variant_get`
expression to successfully cast a value from the `variant_value` to the target
type.
-For example, if `typed_value` is of type `int64`, then the string “123” might
be contained in `variant_value`, which would not be reflected in statistics,
but could be retained by a filter like `where variant_get(col, “$.field”,
“long”) = 123`.
-If `variant_value` is all-null, then the engine can prune pages or row groups
based on `typed_value`.
-This specification is not strict about what values may be stored in
`variant_value` rather than `typed_value`, so it is not safe to skip rows based
on `typed_value` unless the corresponding `variant_value` column is all-null,
or the engine has specific knowledge of the behavior of the writer that
produced the shredded data.
+The series of `tags` arrays `["comedy", "drama"], ["horror", null], ["comedy",
"drama", "romance"], null` would be stored as:
-# Shredding Semantics
+| Array | `value` | `typed_value `|
`typed_value...value` | `typed_value...typed_value` |
+|----------------------------------|-------------|---------------|-----------------------|--------------------------------|
+| `["comedy", "drama"]` | null | non-null | [null,
null] | [`comedy`, `drama`] |
+| `["horror", null]` | null | non-null | [null,
`00`] | [`horror`, null] |
+| `["comedy", "drama", "romance"]` | null | non-null | [null,
null, null] | [`comedy`, `drama`, `romance`] |
+| null | `00` (null) | null |
| |
-Reconstruction of Variant value from a shredded representation is not expected
to produce a bit-for-bit identical binary to the original unshredded value.
-For example, the order of fields in the binary may change, as may the physical
representation of scalar values.
+#### Objects
-In particular, the [Variant Binary Encoding](VariantEncoding.md) considers all
integer and decimal representations to represent a single logical type.
-As a result, it is valid to shred a decimal into a decimal column with a
different scale, or to shred an integer as a decimal, as long as no numeric
precision is lost.
-For example, it would be valid to write the value 123 to a Decimal(9, 2)
column, but the value 1.234 would need to be written to the **variant_value**
column.
-When reconstructing, it would be valid for a reader to reconstruct 123 as an
integer, or as a Decimal(9, 2).
-Engines should not depend on the physical type of a Variant value, only the
logical type.
+Fields of an object can be shredded using a Parquet group for `typed_value`
that contains shredded fields.
-On the other hand, shredding as a different logical type is not allowed.
-For example, the integer value 123 could not be shredded to a string
`typed_value` column as the string "123", since that would lose type
information.
-It would need to be written to the `variant_value` column.
+If the value is not an object, `typed_value` must be null.
-# Reconstructing a Variant
+If the value is a partially shredded object, the `value` must not contain the
shredded fields.
+If shredded fields are present in the variant object, it is invalid and
readers must either fail or use the shredded values.
-It is possible to recover a full Variant value using a recursive algorithm,
where the initial call is to `ConstructVariant` with the top-level fields,
which are assumed to be null if they are not present in the schema.
+Each shredded field in the `typed_value` group is represented as a required
group that contains optional `value` and `typed_value` fields.
+The `value` field stores the value as Variant-encoded `binary` when the
`typed_value` cannot represent the field.
+This layout enables readers to skip data based on the field statistics for
`value` and `typed_value`.
Review Comment:
I'm OK with this, just wanted to make sure this was discussed it seems no
one else has raised concerns on the dev list.
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