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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple style='word-wrap:break-word'><div class=WordSection1><p class=MsoNormal>Well when I run random() I do get a different answer for each run so random behaves as I would expect. I didn’t look that closely at your query with random.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>e.g.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>SELECT random()<o:p></o:p></p><p class=MsoNormal>FROM generate_series(1,100);<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Even if within the same row, the random numbers are different:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>SELECT random(), random()<o:p></o:p></p><p class=MsoNormal>FROM generate_series(1,10);<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>If you were doing random()::integer as input into ST_GeneratePoints, I thought maybe that was a typo on your end. Then your random number would only be 0 or 1, which is not that random.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>So if you really were doing ST_GeneratePoints(geom, random()::integer) then that would explain why you got much less than random results with ST_GeneratePoints.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><o:p> </o:p></p><div style='border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt'><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b>From:</b> Brent Wood <pcreso@yahoo.com> <br><b>Sent:</b> Saturday, November 18, 2023 1:29 AM<br><b>To:</b> Regina Obe <lr@pcorp.us>; PostGIS Users Discussion <postgis-users@lists.osgeo.org><br><b>Subject:</b> Re: [postgis-users] Generating new random points throughout an update<o:p></o:p></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'>Hi Regina,<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'>The seed was an int generated from random(), so I'd expected to generate a different result every time. This didn't happen.<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'>Do I understand that if I omit the seed, I'll get a different point each time by default?<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'>Thanks,<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'> Brent <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div></div><div id="ydp281dcc1fyahoo_quoted_0556156328"><div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>On Saturday, November 18, 2023 at 06:01:37 PM GMT+13, Regina Obe <<a href="mailto:lr@pcorp.us">lr@pcorp.us</a>> wrote: <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'><o:p> </o:p></span></p></div><div><div id=ydp281dcc1fyiv3900421105><div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>If you want the answer different each time, you don’t want to feed a seed to ST_GeneratePoints. <o:p></o:p></span></p><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>The seed argument was added because some people wanted to generate the same answer for each run.<o:p></o:p></span></p><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'><a href="https://postgis.net/docs/ST_GeneratePoints.html" target="_blank">https://postgis.net/docs/ST_GeneratePoints.html</a> (note the sentence: The optional </span><code><span style='font-size:10.0pt;color:#26282A'>seed</span></code><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> is used to regenerate a deterministic sequence of points, and must be greater than zero.)<o:p></o:p></span></p><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p><div style='border:none;border-left:solid windowtext 1.5pt;padding:0in 0in 0in 4.0pt;border-color:currentcolor currentcolor currentcolor blue'><div id=ydp281dcc1fyiv3900421105yqt47953><div><div style='border:none;border-top:solid windowtext 1.0pt;padding:3.0pt 0in 0in 0in;border-color:currentcolor currentcolor'><p class=ydp281dcc1fyiv3900421105msonormal><b><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>From:</span></b><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> postgis-users <<a href="mailto:postgis-users-bounces@lists.osgeo.org">postgis-users-bounces@lists.osgeo.org</a>> <b>On Behalf Of </b>Brent Wood via postgis-users<br><b>Sent:</b> Friday, November 17, 2023 11:53 PM<br><b>To:</b> PostGIS Users Discussion <<a href="mailto:postgis-users@lists.osgeo.org">postgis-users@lists.osgeo.org</a>><br><b>Cc:</b> Brent Wood <<a href="mailto:pcreso@yahoo.com">pcreso@yahoo.com</a>><br><b>Subject:</b> [postgis-users] Generating new random points throughout an update<o:p></o:p></span></p></div></div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>Hopefully someone can help with a problem I'm having.<o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>I have a table with simple linestrings that I need to create a randomly modified version of.<o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>The linestrings represent vessel tracks. I can identify a set of "similar" tracks & create a single "average" linestring that is somewhat representative.<o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>Many of the records don't have a linestring, but for statistical purposes I need to assign a linestring to each - by creating a jittered version of the average linestring so they are not all identical.<o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>The simplest approach I have tried is to use an update with ST_Project() given a random() distance & random() direction applied to each vertex in the average line.<o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>I use the first two vertices with ST_Makeline(), then append a vertex for the third point, as in the SQL below. <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>My problem is that every new line is identical. From some Googled hints, I figure the optimiser has decided to run random() once & re-use the value instead of running the function for every iteration (but I could be wrong!).<o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>Any suggestions as to how I can force a different random result for each record that is updated?<o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>I also tried using ST_GeneratePoints() in a buffer around each point, but need to use something like (random()::int as the seed, and this seems to do exactly the same - valid linestrings are generated, but they are identical, so I'm assuming the seed is not being recalculated for each record.<o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><div><p class=ydp281dcc1fyiv3900421105msonormal><i><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>update events<br>set jittered = ST_MakeLine(<br> (select ST_Project(<br> ST_POINTN(std_track,1),<br> (random()*5000),<br> radians(random()*360))::geometry<br> from std_tow),<br> (select ST_Project(<br> ST_PointN(std_track,2),<br> (random()*5000),<br> radians(random()*360))::geometry<br> from std_tow)<br> );</span></i><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'><o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'>Thanks,<o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> Brent Wood<o:p></o:p></span></p></div></div></div><div><p class=ydp281dcc1fyiv3900421105msonormal><span style='font-size:10.0pt;font-family:"Helvetica",sans-serif;color:#26282A'> <o:p></o:p></span></p></div></div></div></div></div></div></div></div></div></div></body></html>