fix!: fix regression caused by unbalanced partitions and splitting ranges

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

• Performance Regression in TSBS Query Time from v0.9 to v0.10 #5052
• Scan data by time range #4828

What's changed and what's your intention?

As mentioned in #5052 (comment), the SeqScan will push all row groups into the RangeMeta when building the StreamContext.

greptimedb/src/mito2/src/read/range.rs

Lines 258 to 280 in 8bdef77

	fn push_seq_file_ranges(
	num_memtables: usize,
	files: &[FileHandle],
	ranges: &mut Vec<RangeMeta>,
	) {
	// For non append-only mode, each range only contains one file.
	for (i, file) in files.iter().enumerate() {
	let file_index = num_memtables + i;
	if file.meta_ref().num_row_groups > 0 {
	// All row groups share the same time range.
	let row_group_indices = (0..file.meta_ref().num_row_groups)
	.map(|row_group_index| RowGroupIndex {
	index: file_index,
	row_group_index: row_group_index as i64,
	})
	.collect();
	ranges.push(RangeMeta {
	time_range: file.time_range(),
	indices: smallvec![file_index],
	row_group_indices,
	num_rows: file.meta_ref().num_rows as usize,
	});
	} else {

Then SeqScan builds a source for each row group index. This causes the merge reader to load all row groups of each file in build_sources() by iterating the row_group_indices.

greptimedb/src/mito2/src/read/seq_scan.rs

Lines 368 to 379 in 8bdef77

	fn build_sources(
	stream_ctx: &Arc<StreamContext>,
	part_range: &PartitionRange,
	compaction: bool,
	part_metrics: &PartitionMetrics,
	range_builder_list: Arc<RangeBuilderList>,
	sources: &mut Vec<Source>,
	) {
	// Gets range meta.
	let range_meta = &stream_ctx.ranges[part_range.identifier];
	sources.reserve(range_meta.row_group_indices.len());
	for index in &range_meta.row_group_indices {

This PR fixes several issues in SeqScan related to #5052

• Avoids pushing row group indices before split parts. It may cause the merge reader to merge all row groups of the same file, which is unexpected
  • Before this PR, compaction will load all row groups into memory. This is a regression introduced in feat: Seq scanner scans data by time range #4809
• Adds the number of row groups to the SourceIndex so we can know how to split the source without relying on the FileMeta.
• Makes the number of rows in partitions close to each other if possible and doesn't always create target_partitions partitions so the query engine can repartition inputs.

After this PR, we store the row group number in SourceIndex so we can always set the row group index to ALL_ROW_GROUPS when building the StreamContext. Then build_sources() will read a source for the whole file instead of a list of sources for all row groups.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

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Copilot reviewed 19 out of 19 changed files in this pull request and generated no suggestions.

[codecov]

Codecov Report

Attention: Patch coverage is 70.92732% with 116 lines in your changes missing coverage. Please review.

Project coverage is 83.79%. Comparing base (dc83b0a) to head (51e8cae).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5090      +/-   ##
==========================================
- Coverage   84.03%   83.79%   -0.24%     
==========================================
  Files        1162     1167       +5     
  Lines      216767   217534     +767     
==========================================
+ Hits       182153   182292     +139     
- Misses      34614    35242     +628     

[killme2008]

LGTM

[v0y4g3r]

LGTM