STILTS-TGROUP(1)

📦
version 3.5.4 · 3 versions
, in ubuntu stonking

✨ STILTS-TGROUP ✨

okay so bestie, gather round, because this is stilts-tgroup — the Stilts command that calculates aggregate functions on groups of rows. group chat for your data fr fr 💅

🌸 NAME 🌸

stilts-tgroup — calculates aggregate functions on groups of rows. that's the whole vibe, that's the assignment. 🎀

✨ SYNOPSIS ✨

stilts tgroup [ifmt=<in-format>] [istream=true|false] [in=<table>]
              [icmd=<cmds>] [ocmd=<cmds>]
              [omode=out|meta|stats|count|checksum|cgi|discard|topcat|samp|tosql|gui]
              [out=<out-table>] [ofmt=<out-format>] [keys=<expr> ...]
              [aggcols=<expr>;<aggregator>[;<name>] ...]
              [runner=sequential|parallel|parallel<n>|partest]
              [sort=true|false] [cache=true|false]

🦋 DESCRIPTION 🦋

so what does tgroup actually do, you ask? it identifies groups of rows in a table based on the values in a given column or columns, and then it calculates statistical quantities or otherwise collapses down the multiple values from other columns into single values representing each group. basically it does the exact same job as a SELECT ... GROUP BY statement with aggregate functions in ADQL/SQL. iconic behaviour, honestly. 👑

the keys parameter defines how input rows get grouped, and the aggcols parameter defines what quantities to aggregate from the rows in each group. keys specifies one or more values (column names or expresssions) that must be the same for rows grouped together, while aggcols specifies zero or more columns to be added based on the content of rows in each group. so the output table ends up with one column for each entry in keys and one column for each entry in aggcols, and it has one row for each group identified. slay. ✨

this command can therefore be used to count rows or calculate statistical quantities per group. a whole bunch of statistical aggregation methods are provided, like mean, median, minimum, maximum etc. and for more specialised requirements — say, quantiles or custom statistics — you can also use the array aggregators, which generate an array containing all of the values in the group, and then operate on the resulting column using one of the functions in the Arrays class. the way that's actually so flexible?? 🤩

by way of comparison, the tgroup invocation:

stilts tgroup in=t keys="year detector" aggcols="0;count;num gmag;min;min_gmag gmag;mean"

corresponds roughly to the ADQL query:

SELECT COUNT(*) AS num, MIN(gmag) AS min_gmag, MEAN(gmag), FROM t GROUP BY year, detector

also!! go check out the tgridmap and tskymap commands, which provide similar functionality where the grouping is over evenly spaced numeric/coordinate values. different flavour, same energy. 🍓

💖 OPTIONS 💖

  • ifmt=<in-format> — specifies the format of the input table as specified by parameter in. the known formats are listed in SUN/256. use this flag if you already know what format your table is in. if it has the special value (auto) — which is the default — then the program will attempt to detect the format of the table automatically. that can't always be done correctly though, and in that case the program will exit with an error explaining which formats were attempted. ngl this parameter is ignored for scheme-specified tables. 🙈
  • istream=true|false — if set true, the input table specified by the in parameter will be read as a stream. you must give the ifmt parameter in this case. depending on the required operations and processing mode, this may cause the read to fail (sometimes it's necessary to read the table more than once, and bestie, that's just not compatible with streaming 😭). it's not normally necessary to set this flag; in most cases the data will be streamed automatically if that's the best thing to do. however, it can sometimes result in less resource usage when processing large files in certain formats (such as VOTable). this parameter is ignored for scheme-specified tables.
  • in=<table> — the location of the input table. it can take one of these forms:
    • a filename.
    • a URL.
    • the special value "-", meaning standard input. in this case the input format must be given explicitly using the ifmt parameter. note that not all formats can be streamed in this way.
    • a scheme specification of the form :<scheme-name>:<scheme-args>.
    • a system command line with either a "<" character at the start, or a "|" character at the end ("<syscmd" or "syscmd|"). this executes the given pipeline and reads from its standard output. this will probably only work on unix-like systems.

in any case, compressed data in one of the supported compression formats (gzip, Unix compress or bzip2) will be decompressed transparently. no cap. 🫶

  • icmd=<cmds> — specifies processing to be performed on the input table as specified by parameter in, before any other processing has taken place. the value of this parameter is one or more of the filter commands described in SUN/256. if more than one is given, they must be separated by semicolon characters (";"). this parameter can be repeated multiple times on the same command line to build up a list of processing steps. the sequence of commands given in this way defines the processing pipeline which is performed on the table.

commands may alternatively be supplied in an external file, by using the indirection character '@'. thus a value of "@filename" causes the file filename to be read for a list of filter commands to execute. the commands in the file may be separated by newline characters and/or semicolons, and lines which are blank or which start with a '#' character are ignored. a backslash character '\fR' at the end of a line joins it with the following line.

  • ocmd=<cmds> — specifies processing to be performed on the output table, after all other processing has taken place. the value of this parameter is one or more of the filter commands described in SUN/256. if more than one is given, they must be separated by semicolon characters (";"). this parameter can be repeated multiple times on the same command line to build up a list of processing steps. the sequence of commands given in this way defines the processing pipeline which is performed on the table.

commands may alternatively be supplied in an external file, by using the indirection character '@'. thus a value of "@filename" causes the file filename to be read for a list of filter commands to execute. the commands in the file may be separated by newline characters and/or semicolons, and lines which are blank or which start with a '#' character are ignored. a backslash character '\fR' at the end of a line joins it with the following line.

  • omode=out|meta|stats|count|checksum|cgi|discard|topcat|samp|tosql|gui — the mode in which the result table will be output. the default mode is out, which means the result will be written as a new table to disk or elsewhere, as determined by the out and ofmt parameters. but there are other possibilities, which correspond to uses to which a table can be put other than outputting it — like displaying metadata, calculating statistics, or populating a table in an SQL database. for some values of this parameter, additional parameters (<mode-args>) are required to determine the exact behaviour.

the possible values are:

  • out
  • meta
  • stats
  • count
  • checksum
  • cgi
  • discard
  • topcat
  • samp
  • tosql
  • gui

use the help=omode flag or see SUN/256 for more information. iykyk 💅

  • out=<out-table> — the location of the output table. this is usually a filename to write to. if it's equal to the special value "-" (the default), the output table will be written to standard output.
this parameter must only be given if omode has its default value of "out".
  • ofmt=<out-format> — specifies the format in which the output table will be written (one of the ones in SUN/256 — matching is case-insensitive and you can use just the first few letters). if it has the special value "(auto)" — the default — then the output filename will be examined to try to guess what sort of file is required, usually by looking at the extension. if it's not obvious from the filename what output format is intended, an error will result. no guessing games, it just refuses. 💀
this parameter must only be given if omode has its default value of "out".
  • keys=<expr> ... — list of one or more space-separated words defining the groups within which aggregation should be done. each word can be a column name or an expression using the expression language. each expression will appear as one of the columns in the output table. this list corresponds to the contents of an ADQL/SQL GROUP BY clause. periodt. 🌈
  • aggcols=<expr>;<aggregator>[;<name>] ... — defines the aggregate quantities to be calculated for each group of input rows. each quantity is defined by one entry in this list; entries are space-separated, or can be given by multiple instances of this parameter on the command line.

each entry is composed of two or three tokens, separated by semicolon (";") characters:

  • <expr>: (required) column name, or expression using the expression language, for the quantity to be aggregated
  • <aggregator>: (required) aggregation method
  • <name>: (optional) name of output column; if omitted, a name based on the <expr> value will be used

the available <aggregator> values are as follows:

  • count: counts the number of rows
  • ngood: counts the number of non-blank items
  • sum: the sum of all the combined values per bin
  • mean: the mean of the combined values
  • median: the median
  • stdev: the sample standard deviation of the combined values
  • stdev-pop: the population standard deviation of the combined values
  • max: records the maximum value
  • min: records the minimum value
  • array: collects all non-blank values into an array
  • array-withblanks: collects all values into an array; blank values are represented as zero for integers
  • count-long: counts the number of rows, works for >2 billion
  • ngood-long: counts the number of non-blank items, works for >2 billion
  • Q.nnn — the quantile nnn, bestie 💅 so like Q.05? that's the fifth percentile, periodt.
  • runner=sequential|parallel|parallel<n>|partest — this one picks the threading glow-up, i.e. how much of the processing happens in parallel. Right now the options are:
    • sequential — runs using only a single thread. one girl, one job, no entourage 🧚
    • parallel — runs using multiple threads for large tables, and the parallelism is given by the number of available processors
    • parallel<n> — same deal, multiple threads for large tables, but the parallelism is the supplied value <n> that you picked yourself 💁
    • partest — runs using multiple threads even when tables are small. this one is only intended for testing purposes, so don't be out here using it for real life 🙈

Using parallel processing can speed up execution considerably — highkey iconic. however, depending on the I/O operations required, it can also slow it down by disrupting patterns of disk access. 😭 so here's the tea: if the content of a file is on a solid state disk, or is already in cache — for instance because a similar command has been run recently — then parallel will probably be faster. but if the data is being read directly from a spinning disk, for instance because the file is too large to fit in RAM, then sequential or parallel<n> with a small <n> may be faster. ✨

The value of this parameter should make only very tiny differences to the output table. If you notice significant discrepancies please report them. fr fr, they want to know. 🫶

  • sort=true|false — decides whether an attempt gets made to sort the output table by the values of the keys expressions. This may not be possible if no sort order is defined on the keys. ngl.

In most cases such sorting will be a small overhead on the rest of the work done by this task, so the default is true — but if ordering by key is not useful you may save some resources by setting it false. and just so you know, if no sorting is done, the output row order is undefined. 👀

  • cache=true|false — decides whether the results of the aggregation operation will be cached in random-access storage before output. This is set true by default, since accessing rows of the calculated table may be somewhat expensive, and most uses of the results will need all of the cells. But if you anticipate making only a small number of accesses to the output table cells, it could be more efficient to set this false. she's flexible like that 💖

🌈 SEE ALSO 🌈

If the package stilts-doc is installed, the full documentation SUN/256 is available in HTML format:

file:///usr/share/doc/stilts/sun256/index.html

💜 VERSION 💜

STILTS version 3.5.4-debian

This is the Debian version of Stilts, which lack the support of some file formats and network protocols. For differences see:

file:///usr/share/doc/stilts/README.Debian
ok bestie, important: this Debian version is missing support for some file formats and network protocols — that is not a vibe, that is just a fact, so check that Debian readme before you start drama 💀

🎀 AUTHOR 🎀

Mark Taylor (Bristol University)

and that's a wrap on STILTS-TGROUP(1) — this page is from STILTS-TGROUP(1), marinating since Mar 2017, and it's the Debian flavour of the 3.5.4-debian era. go be the main character of your own tables, bestie 💅✨🦋

📦 where this page comes from