2016-11-27 17:50 GMT+01:00 Christian Convey <christian.con...@gmail.com>:
> From looking at other databases' docs, it seems like the behavior of > various JSON-related operators / functions are described partially in terms > of a "json path expression": > > * In Oracle, "JSON_TABLE", "JSON_exists_column", "JSON_value_column": [1] > * In MySQL: [2] > * In DB2: [3] > * In MS SQL Server: [4] > * (Whatever the Standards committee will end up producing.) > > If I'm correctly understanding the situation, It sounds like we have two > big unknowns: > > (a) The exact syntax/semantics of JSON path searching, especially w.r.t. > corner cases and error handling, and > > (b) The syntax/semantics of whatever SQL operators / functions are > currently defined in terms of (a). E.g., "JSON_TABLE". > > If that's correct, then what do you guys think about us taking the > following incremental approach? > > Step 1: I'll dig into the implementations described above, to see what's > similar and different between the JSON-path-expression syntax and semantics > offered by each. I then report my findings here, and we can hopefully > reach a consensus about the syntax/semantics of PG's json-path-expression > handling. > > Step 2: I submit a patch for adding a new function to "contrib", which > implements the JSON-path-expression semantics chosen in Step 1. The > function will be named such that people won't confuse it with any > (eventual) SQL-standard equivalent. > > Step 3: PG developers can, if they choose, start defining new JSON > operator / functions, and/or port existing JSON-related functions, in terms > of the function created in Step 2. > > I see the following pros / cons to this approach: > > Pro: It gives us a concrete start on this functionality, even though we're > not sure what's happening with the SQL standard. > > Pro: The risk of painting ourselves into a corner is relatively low, > because we're putting the functionality in "contrib", and avoid function > names which conflict with likely upcoming standards. > > Pro: It might permit us to give PG users access to JSONPath -like > functionality sooner than if we wait until we're clear on the ideal > long-term interface. > Incremental work is great idea - I like this this style. Instead contrib, you can use public repository on github. Minimally for first stage is better to live outside core - you are not restricted by PostgreSQL development process. When your code will be stabilized, then you can go to commitfest. I believe so we need good JSON support. The XML support helps to PostgreSQL lot of, JSON will be great too. > > Con: "JSON path expression" is a recurring them in the *grammars* of > user-facing operators in [1], [2], [3], and [4]. But it doesn't > necessarily follow that the function implemented in Step 2 will provide > useful infrastructure for PG's eventual implementations of "JSON_TABLE", > etc. > We can implement subset only - our XPath based on libxml2 does it too. The good target is support of usual examples on the net. Regards Pavel > > - Christian > > [1] https://docs.oracle.com/database/121/SQLRF/functions092.htm#SQLRF56973 > [2] https://dev.mysql.com/doc/refman/5.7/en/json-path-syntax.html > [3] http://www.ibm.com/support/knowledgecenter/ssw_ > ibm_i_72/db2/rbafzjsonpath.htm > [4] https://msdn.microsoft.com/en-us/library/mt577087.aspx >
