issue_comments: 813134072

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https://github.com/simonw/datasette/issues/1293#issuecomment-813134072 https://api.github.com/repos/simonw/datasette/issues/1293 813134072 MDEyOklzc3VlQ29tbWVudDgxMzEzNDA3Mg== 9599 2021-04-05T01:18:37Z 2021-04-05T01:18:37Z OWNER

Had a fantastic suggestion on the SQLite forum: it might be possible to get what I want by interpreting the opcodes output by explain select ....

Copying the reply I posted to this thread:

That's really useful, thanks! It looks like it might be possible for me to reconstruct where each column came from using the explain select output.

Here's a complex example: https://calands.datasettes.com/calands?sql=explain+select%0D%0A++AsGeoJSON%28geometry%29%2C+*%0D%0Afrom%0D%0A++CPAD_2020a_SuperUnits%0D%0Awhere%0D%0A++PARK_NAME+like+%27%25mini%25%27+and%0D%0A++Intersects%28GeomFromGeoJSON%28%3Afreedraw%29%2C+geometry%29+%3D+1%0D%0A++and+CPAD_2020a_SuperUnits.rowid+in+%28%0D%0A++++select%0D%0A++++++rowid%0D%0A++++from%0D%0A++++++SpatialIndex%0D%0A++++where%0D%0A++++++f_table_name+%3D+%27CPAD_2020a_SuperUnits%27%0D%0A++++++and+search_frame+%3D+GeomFromGeoJSON%28%3Afreedraw%29%0D%0A++%29&freedraw=%7B%22type%22%3A%22MultiPolygon%22%2C%22coordinates%22%3A%5B%5B%5B%5B-122.42202758789064%2C37.82280243352759%5D%2C%5B-122.39868164062501%2C37.823887203271454%5D%2C%5B-122.38220214843751%2C37.81846319511331%5D%2C%5B-122.35061645507814%2C37.77071473849611%5D%2C%5B-122.34924316406251%2C37.74465712069939%5D%2C%5B-122.37258911132814%2C37.703380457832374%5D%2C%5B-122.39044189453125%2C37.690340943717715%5D%2C%5B-122.41241455078126%2C37.680559803205135%5D%2C%5B-122.44262695312501%2C37.67295135774715%5D%2C%5B-122.47283935546876%2C37.67295135774715%5D%2C%5B-122.52502441406251%2C37.68382032669382%5D%2C%5B-122.53463745117189%2C37.6892542140253%5D%2C%5B-122.54699707031251%2C37.690340943717715%5D%2C%5B-122.55798339843751%2C37.72945260537781%5D%2C%5B-122.54287719726564%2C37.77831314799672%5D%2C%5B-122.49893188476564%2C37.81303878836991%5D%2C%5B-122.46185302734376%2C37.82822612280363%5D%2C%5B-122.42889404296876%2C37.82822612280363%5D%2C%5B-122.42202758789064%2C37.82280243352759%5D%5D%5D%5D%7D

It looks like the opcodes I need to inspect are OpenRead, Column and ResultRow.

OpenRead tells me which tables are being opened - the p2 value (in this case 51) corresponds to the rootpage column in sqlite_master here: https://calands.datasettes.com/calands?sql=select+*+from+sqlite_master - it gets assigned to the register in p1.

The Column opcodes tell me which columns are being read - p1 is that table reference, and p2 is the cid of the column within that table.

The ResultRow opcode then tells me which columns are used in the results. 15 16 means start at the 15th and then read the next 16 columns.

I think this might work!

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