Improve the performance of metadata scanning during hypertable
expansion.
When a hypertable is expanded to include all children chunks, only the
chunks that match the query restrictions are included. To find the
matching chunks, the planner first scans for all matching dimension
slices. The chunks that reference those slices are the chunks to
include in the expansion.
This change optimizes the scanning for slices by avoiding repeated
open/close of the dimension slice metadata table and index.
At the same time, related dimension slice scanning functions have been
refactored along the same line.
An index on the chunk constraint metadata table is also changed to
allow scanning on dimension_slice_id. Previously, dimension_slice_id
was the second key in the index, which made scans on this key less
efficient.
c1cf067c4f