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Data Organization

The SDSS Catalog Data Model

The SDSS catalog data is stored in a commercial relational database management system (DBMS) - Microsoft's SQL Server. The data is therefore organized in several 2-dimensional tables. The tables and their relationships to each other are referred to as the schema in database jargon.

Diagrammatic view of the DR6 schema

There are 3 different types of data in the tables - imaging data is in the photo group of tables, spectroscopic and tiling data is in the spectro tables, and other data such as documentation or other information about the photo and spectro data, i.e. metadata, is in the meta tables. Some tables are also created specifically for speef or convenience, for example the SpecPhotoAll table, which contains a pre-computed join of relevant fields in the PhotoObjAll and SpecObjAll tables.

The important tables are described below, along with the views that are currently defined on each table. A view is a subset of the corresponding table that can be used instead of the table - in other words it is a virtual table. A view is usually faster than using the base table, since it only loads a subset of the objects, but more importantly, the views we have defined on the tables select only the objects that are important for science, and they filter out non-science objects such as sky, QA or defective observations. As such, even though we list the base tables for completeness below, in the vast majority of the cases, you should use the views defined on the tables instead of the tables themselves, e.g. use the PhotoObj and SpecObj views for science instead of the PhotoObjAll and SpecObjAll tables.

BESTDR6, TARGDR6 and other databases

There are two main datasets in the SDSS catalog archive server - the BEST and TARGET datasets that are contained in separate databases. Each contains different reprocessings of the same raw data:

• the BEST dataset is contained in the BESTDR6 database, and represents the latest, greatest calibration of the raw data. BESTDR6 is the default database for queries. Other databases must be explicitly specified (see below for sample syntax).
• The TARGET dataset is contained in the TARGDR6 database, and represents a snapshot of the data as it was when the target selection algorithm was run on it, i.e. it is the calibration upon which the spectroscopic targets were chosen.  Preserving this data version is important for doing science with the data.

Both databases have the identical schema (tables), but different data.  The BESTDR6 database also contains the spectroscopic and tiling data, whereas the TARGDR6 database only contains imaging data.  The vast majority of queries are run on the BESTDR6 database.

To choose a database other than the default BESTDR6 in your query, you must specify it as <dbname>..<tablename>, e.g., TARGDR6..PhotoObj:

	SELECT TOP 10 objid,ra,dec FROM TARGDR6..PhotoObj

Please see the SQL Intro page for further help with SQL queries.

The Hierarchical Triangular Mesh (HTM)

We have build a spatial indexing scheme called the Hierarchical Triangular Mesh (HTM) that spatially decomposes the region of the sky that is covered by the SDSS data and enables much faster spatial searches of the data by coordinate cuts.

Database Indices

In addition to the HTM, which is an overall indexing scheme for multidimensional spatial data, the DBMS itself has the capability to define indices for fast searches on each table. We have defined indices on all the major tables.

An index is a tree representation of a subset of the columns in a table that enables much faster searches of the table (compared to sequential scans of the table data) when constraints involving those columns are included in the query. All tables have an index on their primary key (unique row identifier), but the larger tables have indexes in addition to the primary key index. In all there are 3 kinds of indices:

• Primary Key Index - index on the unique primary key of a table.
• Foreign Key Index - index on a foreignkey of a table, i.e. a column that is a primary key of another table.
• Covering Index - an index that covers one or more columns of a table. This is a combined index on those fields, so it speeds up searches if any of those fields are included in the WHERE clause.

Click here to view a table of all the current indices defined on the data.

Imaging (Photo) Data Tables

The important tables and views are described below. For each table, the views and indices are defined on that table are described in brief. For more information, please see the Table Descriptions help page as well as the links to the Glossary entries that are incorporated in the table listing below.

1. PhotoObjAll - By far the largest table in the database, PhotoObjAll contains the 100+ parameters for each imaging (photo) object. For most of these parameters, there are actually 5 rows each, one for each wavelength band. This table includes data on all photo objects, not just science objects, hence the name PhotoObjAll. The view of this table that includes only science objects and excludes sky and other unknown objects is the PhotoObj view. The PhotoObjAll table is there for completeness, but science queries are usually done on the PhotoObj view.

   PhotoObjAll Views:

   None found

   PhotoObjAll Indices:

Index Type	Key or Field List
primary key	objID
foreign key	Field(fieldID)
covering index	mode, cy, cx, cz, htmID, type, flags, status, ra, dec, u, g, r, i, z, rho
covering index	htmID, cx, cy, cz, type, mode, flags, status, ra, dec, u, g, r, i, z, rho
covering index	htmID, run, camcol, field, rerun, type, mode, flags, status, cx, cy, cz, g, r, rho
covering index	field, run, rerun, camcol, type, mode, flags, rowc, colc, ra, dec, u, g, r, i, z
covering index	fieldID, objID, ra, dec, r, type, status, flags
covering index	SpecObjID, cx, cy, cz, mode, type, flags, status, ra, dec, u, g, r, i, z, rho
covering index	cx, cy, cz, htmID, mode, type, flags, status, ra, dec, u, g, r, i, z, rho
covering index	run, mode, type, status, flags, u, g, r, i, z, Err_u, Err_g, Err_r, Err_i, Err_z
covering index	run, camcol, rerun, type, mode, status, flags, ra, dec, fieldID, field, u, g, r, i, z
covering index	run, camcol, field, mode, parentID, q_r, q_g, u_r, u_g, isoA_r, isoB_r, fiberMag_u, fiberMag_g, fiberMag_r, fiberMag_i, fiberMag_z
covering index	run, camcol, type, mode, cx, cy, cz
covering index	ra, [dec], type, mode, flags, u, g, r, i, z, psfMag_u, psfMag_g, psfMag_r, psfMag_i, psfMag_z, primTarget

PhotoTag - This is a vertical partition of the PhotoObjAll table, and contains only those columns that are most often requested. Due to the smaller size of each row in the table, many more rows can be loaded into the memory cache at one time, hence searches on the PhotoTag table are much faster than searches on PhotoObjAll. Whenever possible, use the PhotoTag table instead of PhotoObjAll or PhotoObj.

PhotoTag Indices:

Index Type	Key or Field List
primary key	objID
foreign key	PhotoObjAll(objID)
foreign key	Field(fieldID)
covering index	mode, cy, cx, cz, htmID, type, flags, status, ra, [dec], modelMag_u, modelMag_g, modelMag_r, modelMag_i, modelMag_z, [size]
covering index	htmID, cx, cy, cz, type, mode, flags, status, ra, [dec], modelMag_u, modelMag_g, modelMag_r, modelMag_i, modelMag_z, [size]
covering index	field, run, rerun, camcol, type, mode, flags, ra, [dec], modelMag_u, modelMag_g, modelMag_r, modelMag_i, modelMag_z
covering index	fieldID, objID, ra, [dec], modelMag_r, type, status, flags
covering index	SpecObjID, cx, cy, cz, mode, type, flags, status, ra, [dec], modelMag_u, modelMag_g, modelMag_r, modelMag_i, modelMag_z, [size]
covering index	cx, cy, cz, htmID, mode, type, flags, status, ra, [dec], modelMag_u, modelMag_g, modelMag_r, modelMag_i, modelMag_z, [size]
covering index	run, mode, type, status, flags, modelMag_u, modelMag_g, modelMag_r, modelMag_i, modelMag_z, modelMagErr_u, modelMagErr_g, modelMagErr_r, modelMagErr_i, modelMagErr_z
covering index	run, camcol, field, rerun, type, mode, status, flags, ra, [dec], fieldID, modelMag_u, modelMag_g, modelMag_r, modelMag_i, modelMag_z
covering index	ra, [dec], type, mode, flags, modelMag_u, modelMag_g, modelMag_r, modelMag_i, modelMag_z, psfMag_u, psfMag_g, psfMag_r, psfMag_i, psfMag_z, primTarget

Segment - This table contains the basic parameters associated with a Segment, which is a unit of data corresponding to a single camera column within a chunk.

Segment Indices:

Index Type	Key or Field List
primary key	segmentID
foreign key	Chunk(chunkId)
foreign key	StripeDefs(stripe)

Field - This table contains all the measured parameters of each imaging field, along with relevant summary statistics, and astrometric and photometric information.

Field Indices:

Index Type	Key or Field List
primary key	fieldID
foreign key	Segment(segmentID)
covering index	field, camcol, run, rerun
covering index	run, camcol, field, rerun

Chunk - A chunk is a contiguous section of imaging data within a stave (stripe for the south). It consists of complete North and South strips between lower and upper mu limits. A chunk is composed of a set of touching but non-overlapping primary segments (or parts of segments).

Chunk Indices:

Index Type	Key or Field List
primary key	chunkID
foreign key	StripeDefs(stripe)

PhotoProfile - This table contains the light profiles of SDSS photo objects.

PhotoProfile Indices:

Index Type	Key or Field List
primary key	objID, bin, band
foreign key	PhotoObjAll(objID)

FieldProfile - This table contains the light profiles of SDSS field objects.

FieldProfile Indices:

Index Type	Key or Field List
primary key	fieldID, bin, band
foreign key	Field(fieldID)

Neighbors - SDSS objects within 0.5 arcmins and their match parameters are stored here. Make sure to filter out unwanted PhotoObj, like secondaries.

Neighbors Indices:

Index Type	Key or Field List
primary key	objID, NeighborObjID
foreign key	PhotoObjAll(objID)

Match - This table contains PhotoObj (primary and secondary object) pairs from different runs (observations) that probably are the same object. In effect, this table records multiple observations of each object.

Match Indices:

Index Type	Key or Field List
primary key	objID1, objID2
foreign key	PhotoObjAll(objID)
foreign key	MatchHead(objid)
covering index	matchHead

MatchHead - Thhis table records the canonical object from each match cluster, and cluster statistics. The observations for an object (as recorded in the Match table) form a cluster named by the minimum objID in the cluster. MatchHead has summary information about the cluster keyed by the objID.

MatchHead Indices:

Index Type	Key or Field List
primary key	objID
foreign key	PhotoObjAll(objID)

First, Rosat and Usno - These tables contain matches between the FIRST, ROSAT and USNO survey objects and SDSS.

Spectro/Tiling Data Tables

1. PlateX - This table contains data as exported (the X is for exported) from a given plate used for spectroscopic observations. Each plate has 640 observed spectra and hence 640 corresponding entries in SpecObjAll.

   PlateX Indices:

   Index Type	Key or Field List
   primary key	plateID
   foreign key	TileAll(tile)
   covering index	htmID, ra, dec, cx, cy, cz

2. SpecObjAll - This is a base table containing ALL the spectroscopic information, including a lot of duplicate and bad data. Use the SpecObj view instead (see below), which has the data properly filtered for cleanliness.

   SpecObjAll Views:

   None found

   SpecObjAll Indices:

   Index Type	Key or Field List
   primary key	specObjID
   foreign key	PlateX(plateID)
   covering index	htmID, ra, dec, cx, cy, cz, sciencePrimary
   covering index	BestObjID, objType, objTypeName, sciencePrimary, specClass, htmID, ra, dec, fiberMag_u, fiberMag_g, fiberMag_r, fiberMag_i, fiberMag_z
   covering index	specClass, zStatus, zWarning, z, sciencePrimary, primTarget, secTarget, plateId, bestObjID, targetObjId, htmID, ra, dec
   covering index	targetObjID, objType, objTypeName, sciencePrimary, specClass, htmID, ra, dec, fiberMag_u, fiberMag_g, fiberMag_r, fiberMag_i, fiberMag_z
   covering index	ra, [dec], specClass, plate, tile, z, zErr, zConf, fiberMag_r, primTarget, sciencePrimary, plateID, bestObjID

3. SpecLineAll - A compendium of all the spectral lines found in all the spectroscopic objects in the SpecObjAll table. Contains all the parameters measured for each spectral line. There is a SpecLine view of this table that contains only the lines that have been measured.

   SpecLineAll Views:

   None found

   SpecLineAll Indices:

   Index Type	Key or Field List
   primary key	specLineID
   foreign key	SpecObjAll(specObjID)
   covering unique index	specobjID, specLineID

4. SpecLineIndex - Precomputed spectral line indices. These are combinations of spectral line intensities used to determine various properties of galaxies, like age or metallicity.

   SpecLineIndex Views:

   None found

   SpecLineIndex Indices:

   Index Type	Key or Field List
   primary key	specLineIndexID
   foreign key	SpecObjAll(specObjID)
   covering unique index	specobjID, speclineindexID

5. TileAll - Contains information about individual tiles on the sky.

   TileAll Views:

   None found

   TileAll Indices:

   Index Type	Key or Field List
   primary key	tile
   foreign key	TilingRun(tileRun)
   covering unique index	tileRun, tile
   covering index	htmID, racen, deccen, cx, cy, cz

6. TiledTargetAll - This table stores information that keeps track of why a Target was assigned to a Tile.

   TiledTargetAll Views:

   None found

   TiledTargetAll Indices:

   Index Type	Key or Field List
   primary key	targetID, tile
   foreign key	TileAll(tile)
   covering unique index	tile, targetID
   covering index	htmID, ra, dec, cx, cy, cz, objtype
   covering unique index	objType, targetID, tile

7. TilingGeometry - This table contains geometrical information about tiling regions, including tiling boundaries. The TileBoundary view serves up the boundaries.

   TilingGeometry Views:

   None found

   TilingGeometry Indices:

   Index Type	Key or Field List
   primary key	tilingGeometryID
   foreign key	TilingRun(tileRun)
   foreign key	StripeDefs(stripe)

8. TilingRun - Contains basic information about each run of the tiling software.
9. TilingInfo - Contains information on what happened to a Target in a run of tiling software.

Metadata and Other Tables

1. DBObjects - An auto-generated documentation table that contains a description of every object (table, view, function, stored procedure) in the database.
2. DBViewCols - An auto-generated table containing a description of every column in every table in the database.
3. DBColumns - An auto-generated table containing a description of every column in every view in the database.
4. SpecPhotoAll - The combined spectro and photo parameters of an object in SpecObjAll. This is a precomputed join between the PhotoObjAll and SpecObjAll tables. The photo attibutes included cover about the same as PhotoTag. The table also includes certain attributes from the Tile table.

   SpecPhotoAll Views:

   None found

   SpecPhotoAll Indices:

   Index Type	Key or Field List
   primary key	specObjID
   foreign key	SpecObjAll(specObjID)
   covering index	objID, sciencePrimary, specClass, z, targetObjid, targetId
   covering index	targetObjID, sciencePrimary, specClass, z, objid, targetId
   covering index	targetID, sciencePrimary, specClass, z, objid, targetObjId