ESQL: Reserve memory TopN (#134235)

[nik9000]

Tracks the more memory that's involved in topn.

Lucene doesn't track memory usage for TopN and can use a fair bit of it. Try this query:

FROM big_table
| SORT a, b, c, d, e
| LIMIT 1000000
| STATS MAX(a)

We attempt to return all million documents from lucene. Is we did this with the compute engine we're track all of the memory usage. With lucene we have to reserve it.

In the case of the query above the sort keys weight 8 bytes each. 40 bytes total. Plus another 72 for Lucene's FieldDoc. And another 40 at least for copying to the values to FieldDoc. That totals something like 152 bytes a piece. That's 145mb. Worth tracking!

Esql Engine TopN

Esql does track memory for topn, but it doesn't track the memory used by the min heap itself. It's just a big array of pointers. But it can get very big!

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