sql: optimize query to populate crdb_internal.table_row_statistics

The query to populate the `crdb_internal.table_row_statistics` virtual
table has been rewritten to avoid scanning the `system.table_statistics`
table twice.

Before this commit the query plan was:

    • group (hash)
    │ group by: tableID
    │
    └── • hash join
        │ actual row count: 515,822
        │ equality: (tableID, createdAt) = (tableID, max)
        │
        ├── • scan
        │     table: table_statistics@primary
        │     spans: FULL SCAN
        │
        └── • group (streaming)
            │ group by: tableID
            │ ordered: +"tableID"
            │
            └── • scan
                  table: table_statistics@primary
                  spans: FULL SCAN

Now it is:

    • distinct
    │ distinct on: tableID
    │ order key: tableID
    │
    └── • sort
        │ order: +"tableID",-"createdAt",-"rowCount"
        │ already ordered: +"tableID"
        │
        └── • scan
              table: table_statistics@primary
              spans: FULL SCAN

The `crdb_internal.table_row_statistics` table is used to populate the
`estimated_row_count` column in the output of `SHOW TABLES`. In
pathological cases where there a many rows in `system.table_statistics`,
this new query makes `SHOW TABLES` significantly faster. In a test of
mine with ~1.1 million rows in `system.table_statistics` setup with
[this](https://gist.github.com/mgartner/b72b39901be0d942d5a026054e688a8c),
I observed the latency of `SHOW TABLES` drop by ~50% with the new query,
from ~720ms to ~350ms.

Informs #143438

Release note (performance improvement): `SHOW TABLES` is now faster,
especially in cases where there are many tables, both live and
previously dropped.

Author: mgartner

pkg/sql/crdb_internal.go

@@ -768,15 +768,11 @@ CREATE TABLE crdb_internal.table_row_statistics (
 		// contention on the stats table. We pass a nil transaction so that the AS
 		// OF clause can be independent of any outer query.
 		query := fmt.Sprintf(`
-           SELECT s."tableID", max(s."rowCount")
-             FROM system.table_statistics AS s
-             JOIN (
-                    SELECT "tableID", max("createdAt") AS last_dt
-                      FROM system.table_statistics
-                     GROUP BY "tableID"
-                  ) AS l ON l."tableID" = s."tableID" AND l.last_dt = s."createdAt"
-            AS OF SYSTEM TIME '%s'
-            GROUP BY s."tableID"`, statsAsOfTimeClusterMode.String(&p.ExecCfg().Settings.SV))
+			SELECT DISTINCT ON ("tableID") "tableID", "rowCount"
+			FROM system.table_statistics
+			AS OF SYSTEM TIME '%s'
+			ORDER BY "tableID", "createdAt" DESC, "rowCount" DESC`,
+			statsAsOfTimeClusterMode.String(&p.ExecCfg().Settings.SV))
 		statRows, err := p.ExtendedEvalContext().ExecCfg.InternalDB.Executor().QueryBufferedEx(
 			ctx, "crdb-internal-statistics-table", nil,
 			sessiondata.NodeUserSessionDataOverride,