Example for simple Expr --> SQL conversion

[edmondop]

Which issue does this PR close?

Closes #10524 .

At the time of opening this PR, the example fails with:

thread 'main' panicked at datafusion-examples/examples/plan_to_sql.rs:43:5:
assertion `left == right` failed
  left: "a < Int32(5) OR a = Int32(8)"
 right: "a < 5 OR a = 8"

[edmondop]

Using the expr_to_sql api, we get the following error:

assertion `left == right` failed
  left: "((\"a\" < 5) OR (\"a\" = 8))"
 right: "a < 5 OR a = 8"

[alamb]

Using the expr_to_sql api, we get the following error:

For anyone else following along, this conversation is happening on the ticket: #10524 (comment)

[alamb]

Thanks @edmondop -- this is pretty sweet

It would also be good to add plan_to_sql.rs to the list of examples in https://github.com/apache/datafusion/tree/main/datafusion-examples#single-process

[alamb]

Love it!

[edmondop]

Thanks @edmondop -- this is pretty sweet

It would also be good to add plan_to_sql.rs to the list of examples in main/datafusion-examples#single-process

Examples are now more complete and readme is updated

[yyy1000]

There seems an additional -?

[alamb]

Thank you @edmondop -- this looks like a great start of these examples. I think @yyy1000's comments about the extra - should be addressed but it isn't required.

cc @devinjdangelo and @backkem (given you started this project)

[alamb]

I filed #10550 for hte logical plan version too

[backkem]

Using the expr_to_sql api, we get the following error:

assertion `left == right` failed
  left: "((\"a\" < 5) OR (\"a\" = 8))"
 right: "a < 5 OR a = 8"

The current unparser is somewhat conservative for correctness sake; Always quoting identifiers and adding every set of parentheses possible. If we want to make the generated SQL more succinct, these steps will have to be made "smarter". We'll have to add in the math rules to avoid unneeded parentheses and (likely dialect specific) rules for determining of quoting is needed. Note that the latter likely involves listing out the reserved keywords for each dialect.

[alamb]

Thanks again @edmondop

[alamb]

Using the expr_to_sql api, we get the following error:

assertion `left == right` failed
  left: "((\"a\" < 5) OR (\"a\" = 8))"
 right: "a < 5 OR a = 8"

The current unparser is somewhat conservative for correctness sake; Always quoting identifiers and adding every set of parentheses possible. If we want to make the generated SQL more succinct, these steps will have to be made "smarter". We'll have to add in the math rules to avoid unneeded parentheses and (likely dialect specific) rules for determining of quoting is needed. Note that the latter likely involves listing out the reserved keywords for each dialect.

Thank you for that excellent explanation @backkem . Since this has now come up several times, I filed #10557 to track it