Fix issuable search optimization in PG12

What does this MR do?

Uses a materialized CTE so that our optimization fence actually works.

Recursive CTEs are materialized by default in PG12 but we're not using this recursively here so we use Gitlab::SQL::CTE which handles adding of the MATERIALIZED keyword when needed.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #331089 (closed)

changed milestone to %13.12

added database devopsplan groupproject management severity2 labels

mentioned in commit 93a80b7f

added 1 commit

• 93a80b7f - Fix issuable search optimization in PG12

Compare with previous version

changed the description

marked the checklist item I have included a changelog entry, or it's not needed. (Does this MR need a changelog?) as completed

marked the checklist item I have added/updated documentation, or it's not needed. (Is documentation required?) as completed

marked the checklist item I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?) as completed

marked the checklist item I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?) as completed

marked the checklist item I have self-reviewed this MR per code review guidelines. as completed

marked the checklist item This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines) as completed

marked the checklist item I have followed the style guides. as completed

marked the checklist item I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.) as completed

marked the checklist item I have tested this MR in all supported browsers, or it's not needed. as completed

marked the checklist item I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed. as completed

marked the checklist item Label as security and @ mention @gitlab-com/gl-security/appsec as completed

marked the checklist item The MR includes necessary changes to maintain consistency between UI, API, email, or other methods as completed

marked the checklist item Security reports checked/validated by a reviewer from the AppSec team as completed

added databasereview pending label

added backend label