[8.18] Ensure ordinal builder emit ordinal blocks (#127949) #127980
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Currently, if a field has high cardinality, we may mistakenly disable
emitting ordinal blocks. For example, with 10,000 `tsid` values, we
never emit ordinal blocks during reads, even though we could emit blocks
for 10 `tsid` values across 1,000 positions. This bug disables
optimizations for value aggregation and block hashing.
This change tracks the minimum and maximum seen ordinals and uses them
as an estimate for the number of ordinals. However, if a page contains
`ord=1` and `ord=9999`, ordinal blocks still won't be emitted.
Allocating a bitset or an array for `value_count` could track this more
accurately but would require additional memory. I need to think about
this trade off more before opening another PR to fix this issue
completely.
This is a quick, contained fix that significantly speeds up time-series
aggregation (and other queries too).
The execution time of this query is reduced from 3.4s to 1.9s with 11M documents.
```
POST /_query
{
"profile": true,
"query": "TS metrics-hostmetricsreceiver.otel-default
| STATS cpu = avg(avg_over_time(`metrics.system.cpu.load_average.1m`)) BY host.name, BUCKET(@timestamp, 5 minute)"
}
```
```
"took": 3475,
"is_partial": false,
"documents_found": 11368089,
"values_loaded": 34248167
```
```
"took": 1965,
"is_partial": false,
"documents_found": 11368089,
"values_loaded": 34248167
```
dnhatn
added
:Analytics/ES|QL
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Backports the following commits to 8.18: