Glossary¶

HATS¶

The Hierarchical Adaptive Tile Storage format used by LINCC Frameworks (LSDB, hats-import) for sky-partitioned parquet catalogs. Each partition is one HEALPix pixel at a given Norder. See https://hats.readthedocs.io.

HEALPix¶

A scheme for tiling the sphere into equal-area pixels. Two parameters: Norder (how fine — 0 is coarsest, 29 is the finest order used by HATS) and Npix (the pixel ID within that order).

Norder / Npix¶

A HATS partition is uniquely identified by (Norder, Npix). Norder=5 gives 12,288 pixels over the whole sky; each step in Norder splits every pixel into 4.

Partition¶

A single parquet file (or directory of parquet files) for one (Norder, Npix) of a HATS catalog.

Margin cache¶

A companion catalog that holds a thin border of rows around each partition (typically a few arcsec wide) so a crossmatch on the anchor side never silently misses a match that sits just across a partition boundary.

MOC¶

A Multi-Order Coverage map: a region of sky encoded as a set of HEALPix pixels at possibly mixed orders. Used by surveys to describe footprints (DES, 2MASS, …). acid's IN_MOC(...) predicate filters rows against a MOC.

Anchor catalog¶

The first table in a query's FROM clause. acid partitions its work along the anchor's partitions; coordinates from the anchor drive every XMATCH in the query.

Right catalog¶

A catalog joined to the anchor via XMATCH (or an ordinary equi-join).

XMATCH¶

acid's spherical-distance join operator. Used in ON clauses: JOIN b ON XMATCH(radius_arcsec => 1.0, mode => 'nearest' | 'all').

XMATCH_DISTANCE¶

A SELECT-level function returning the great-circle distance (in arcseconds) between the anchor and a matched right-side row.

IN_MOC¶

A predicate that returns true when a row's sky position lies inside a named MOC footprint.

Refinement tree¶

The mechanism acid uses to align adaptive-Norder catalogs (where different sky regions are partitioned at different Norders). You won't see it in the API; it just makes adaptive queries correct.

_healpix_29¶

A column present in HATS catalogs holding the order-29 pixel ID of each row. acid uses range filters on this column for fast spatial pruning.

point_map.fits¶

A standard HATS file at the catalog root that stores the catalog's own sky-density footprint (an implicit HEALPix image). acid can load this as a MOC for IN_MOC(<alias>, '<catalog_name>').

Row-group pushdown¶

A parquet feature: when a query filters on a column, the parquet reader can skip whole row-groups whose min/max statistics rule them out. acid leans on this heavily for _healpix_29 and column-projection pruning.