fix postgresql string property index

[tarilabs]

See #194

Proposed solution(s)

Taking MLMD's Context string property as example,

S1. modify PostgreSQL index to be ~like:

CREATE INDEX idx_context_property_string_gist ON public.contextproperty USING gist (name gist_trgm_ops, string_value gist_trgm_ops);

or derivatives. (requires first CREATE EXTENSION pg_trgm;)

S2. modify PostgreSQL index to be ~like:

CREATE INDEX idx_context_property_string ON public.contextproperty USING btree (name, is_custom_property, "substring"(string_value, 1, 255));

making it similar to MySQL behaviour, less ideal because would only be partially helpful on queries longer term.

(S3. a third solution would be to drop the idx_..._property_string indexes altogether which seems to be causing the issue, but I don't think is worth considering).

This PR

The scope of this PR is to demonstrate S1 in practice,
or discuss alternative approaches.

/cc @XinranTang could you kindly consult with the team and feedback on this, please?

[tarilabs]

@XinranTang could you kindly take a look, please?

[tarilabs]

@chensun was this possibly been looked into during last summer of code, also ref kubeflow/pipelines#7512 ; thank you in advance for your feedback