PARQUET-2473: Clarify records can not be split across v2 pages or PageIndex

[alamb]

This was sparked by a mailing list discussion: https://lists.apache.org/thread/rd8twnvg4bg3558r507rzpxckcxt5wdn

Several implementors of the Parquet spec have been confused by this point

Notes

There seems to be clear consensus that record boundaries can't span pages in V2 pages or if there is a page index, so let's make that clear in the spec to avoid future confusion

There also seemed to be consensus that Row Groups must start on record boundaries, and that the existing spec language was clear on this point, so I did not propose any changes to that language

https://issues.apache.org/jira/browse/PARQUET-2473

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[alamb]

I also propose expanding the r = 0 terminology to be explicitly repetition_level = 0 to match the conventions in the rest of the documentation

[mapleFU]

LGTM, just a nit

[tustvold]

I wonder if we could instead state that records should not be split across pages, but that readers should tolerate this in the abscence of a page index or v2 data pages, as certain writers historically wrote such data?

[alamb]

I agree with you that splitting pages will make the file less compatible, even though some writers historically did that.

However, I would like to ensure there is consensus on updating the spec to say that writers should do something before changing the spec

[alamb]

Updated to clarify that records can't repeat when an OffsetIndex is present

[pitrou]

Can we unify the row/record terminology? They seem to mean the same thing.

[alamb]

I agree row/record seem to mean the same thing

I double checked and it appears the rest of the file is inconsistent in the terminology as well

For example the repetition level documentation refers to records

parquet-format/src/main/thrift/parquet.thrift

Lines 180 to 192 in 8d59c7d

	/**
	* Representation of Schemas
	*/
	enum FieldRepetitionType {
	/** This field is required (can not be null) and each record has exactly 1 value. */
	REQUIRED = 0;
	/** The field is optional (can be null) and each record has 0 or 1 values. */
	OPTIONAL = 1;
	/** The field is repeated and can contain 0 or more values */
	REPEATED = 2;
	}

but there are several fields that are named num_rows that clearly refer to rows.

parquet-format/src/main/thrift/parquet.thrift

Lines 623 to 631 in 8d59c7d

	/** Number of values, including NULLs, in this data page. **/
	1: required i32 num_values
	/** Number of NULL values, in this data page.
	Number of non-null = num_values - num_nulls which is also the number of values in the data section **/
	2: required i32 num_nulls
	/** Number of rows in this data page. which means pages change on record boundaries (r = 0) **/
	3: required i32 num_rows
	/** Encoding used for data in this page **/
	4: required Encoding encoding

In this PR I followed the term used in the repetition level documentation as I think it is the most relevant.

I will start a conversation on the mailing list about using consistent terminology

[alamb]

Mailing list conversation: https://lists.apache.org/thread/0gxjk4tphxls0gcrc7lt775pf8s7gtz3

Currently the consensus seems to be to use "row"

I will make a follow on PR to change that terminology after this PR is merged.

I would like to retain the "record" terminology here to remain consistent with the current wording in the repetition levels section

[alamb]

#256

[pitrou]

So a row is a record?

[alamb]

yes

[alamb]

I think this PR is now ready to go @wgtmac . Is there anything else we are waiting on?

[wgtmac]

Sorry for missing this. I'll merge it now.

[alamb]

Sorry for missing this. I'll merge it now.

No worries -- thank you!

I'll file a JIRA shortly to improve the spec to use row terminology

[alamb]

Update: here is a PR to consistently use the "row" terminology: #256