Which issue does this PR close?

• Closes Support LEFT JOIN LATERAL #21199.

Rationale for this change

This PR adds support for LEFT join semantics for lateral joins. This is a bit tricky because of how it interacts with compensation for the "count bug". This might be easiest to illustrate with an example; consider this query (Q1):

  SELECT t1.id, sub.cnt
  FROM t1 LEFT JOIN LATERAL (
      SELECT count(*) AS cnt FROM t2 WHERE t2.t1_id = t1.id
  ) sub ON sub.cnt > 0
  ORDER BY t1.id;

The initial decorrelation (Q2) is

  SELECT t1.id, sub.cnt
  FROM t1 LEFT JOIN (
      SELECT count(*) AS cnt, t2.t1_id, TRUE AS __always_true
      FROM t2 GROUP BY t2.t1_id
  ) sub ON t1.id = sub.t1_id

Ignoring the user's original ON clause for now. This initial query is wrong, because t1 rows that don't have a match in t2 will get all-NULL values, not 0 for count(*) of an empty set. This is the "count bug", and we compensate for that by checking for rows when __always_true is NULL, and replacing the agg value with the default for that agg (Q3):

  SELECT t1.id,
         CASE WHEN sub.__always_true IS NULL THEN 0
              ELSE sub.cnt END AS cnt
  FROM (  /* Q2 */  )

Now we just need to handle the user's original ON clause. We can't add this to the rewritten ON clause in Q1, because we don't want the count-bug compensation to fire. But we also can't just add it to the WHERE clause, because we need left join semantics. So we can instead wrap another CASE that re-checks the ON condition and substitutes NULL for every right-side column:

  SELECT t1.id,
         CASE WHEN (cnt > 0) IS NOT TRUE THEN NULL
              ELSE cnt END AS cnt
  FROM (  /* Q3 */  )

What changes are included in this PR?

• Implement lateral left join rewrite as described above
• Update expected tests and add more test cases
• Update documentation

Are these changes tested?

Yes. All new test queries were also run against DuckDB to confirm that both systems produce the same results.

Are there any user-facing changes?

Support for a new feature.