Hey I’m looking for a tech advice: I want to estimate

Question

Hey I’m looking for a tech advice: I want to estimate

the # of rows in a resultset for an OLAP query before acutally running the query on a huge table.
Motivation: return error early if the resultset estimated size is anyway too big for a client to process.

What I’m thinking now is to parse the output of EXLAIN + tune statistics for columns + run analyze more often. Is this going to work?
Or are there better solutions? 🙂

#programming #russian

0

20.02.2024

8 ответов

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Stefanie Janine Stölting

For a table you can use this query: ` SELECT reltuples::bigint AS estimate FROM pg_class WHERE relname = 'tablename' ;

0

9 месяцев назад

Vitali Kotik Автор вопроса

Stefanie Janine Stölting
For a table you can use this query: ` SELECT reltu...

Hey, thanks! It’s not that I need the total # of rows, but the # of rows that correspond to some query (WHERE + GROUP BY) 🙂

0

9 месяцев назад

Stefanie Janine Stölting

Vitali Kotik
Hey, thanks! It’s not that I need the total # of r...

Nope, the query I posted returns the numbers of records estimated. With a complex query there is no chance to estimate the numbers.

0

9 месяцев назад

Vitali Kotik Автор вопроса

Stefanie Janine Stölting
Nope, the query I posted returns the numbers of re...

What is a complex query?

0

9 месяцев назад

Stefanie Janine Stölting

Vitali Kotik
What is a complex query?

Anything containing joins, where, and/or group by. An estimation on such things is like rolling a dice with thousands of sites.

0

9 месяцев назад

Vitali Kotik Автор вопроса

Stefanie Janine Stölting
Anything containing joins, where, and/or group by....

I don’t have joins, but I do have WHERE and GROUP BY. I assumed that with different kinds of stats (https://www.postgresql.org/docs/current/planner-stats.html) planner could do that

0

9 месяцев назад

Stefanie Janine Stölting

Vitali Kotik
I don’t have joins, but I do have WHERE and GROUP ...

Well, read the source code of the planner. It's a very complex thing and it doesn't estimate the count of rows.

0

9 месяцев назад

Kirk · Accepted Answer

There are much better solutions. Depending on the complexity of the query. If it were just one table, I believe there are articles (Cybertec) comes to mind, that explains what DETAILS are used to estimate the number of rows, because what the optimizer does and EXPLAIN does is open source and pretty well documented. I've done something similar to hit a single table for estimated rows from that table, as opposed to using an expensive select count()... when the actual count was NOT really important.

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